U.S. patent application number 11/425067 was filed with the patent office on 2006-12-21 for apparatus and method of producing net-shaped components from alloy sheets.
Invention is credited to Raymond F. Decker, Amit Ghosh.
Application Number | 20060283529 11/425067 |
Document ID | / |
Family ID | 37395927 |
Filed Date | 2006-12-21 |
United States Patent
Application |
20060283529 |
Kind Code |
A1 |
Ghosh; Amit ; et
al. |
December 21, 2006 |
Apparatus and Method of Producing Net-Shaped Components from Alloy
Sheets
Abstract
A method and apparatus for producing ultra-fine grained metal
material sheets. The apparatus molds and rapidly solidifies a metal
material to form a fine grain precursor. The precursor is then
subjected to a series of successive alternating tensile and
compressive strains that alter the grain structure of the precursor
so as to form a ultra fine grained structure in sheet form. The
sheet form may then be subjected to superplastic forming to form a
net shaped article.
Inventors: |
Ghosh; Amit; (Ann Arbor,
MI) ; Decker; Raymond F.; (Ann Arbor, MI) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Family ID: |
37395927 |
Appl. No.: |
11/425067 |
Filed: |
June 19, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60691747 |
Jun 17, 2005 |
|
|
|
Current U.S.
Class: |
148/557 ;
266/103 |
Current CPC
Class: |
C22F 1/10 20130101; C21D
8/0242 20130101; C22F 1/06 20130101; C21D 8/0215 20130101; C22F
1/08 20130101; C22F 1/183 20130101; C22C 1/005 20130101; C22F 1/002
20130101; C21D 7/13 20130101; B21D 13/04 20130101; C22F 1/04
20130101; C21D 8/00 20130101; B21D 13/02 20130101; B22D 17/007
20130101; B21B 3/00 20130101 |
Class at
Publication: |
148/557 ;
266/103 |
International
Class: |
C21D 8/00 20060101
C21D008/00 |
Claims
1. A method of forming a sheet material having a refined grained
structure, the method comprising the steps of: providing a metal
material; molding and rapidly solidifying the metal alloy to form a
fine grain precursor; and imparting plastic deformation to the fine
grain precursor by a combination of alternating tensile strain and
compressive strain to form an ultra fine grain structured sheet
form.
2. The method of claim 1 wherein the step of molding and
solidifying develops a mutliphased microstructure in the fine
grained precursor.
3. The method of claim 2 wherein the multiphased microstructure
includes pinning particles that minimize grain growth.
4. The method of claim 1 wherein the step of imparting plastic
deformation includes the step of storing dislocations in the
microstructure.
5. The method of claim 1 wherein the step of imparting plastic
deformation includes the step of causing the formation of new grain
boundaries having high misorientation suitable for warm forming or
superplastic forming.
6. The method of claim 1 wherein the molding step and the imparting
plastic deformation step are performed in an integrated
apparatus.
7. The method of claim 1 wherein the molding step and the imparting
plastic deformation step are performed by separate machines.
8. The method of claim 1 wherein the molding step includes
semisolid metal injection molding of the metal material.
9. The method of claim 1 wherein the molding step includes one of
extruding of the metal material and twin roll casting of the metal
material.
10. The method of claim 1 wherein the imparting plastic deformation
step includes corrugating the precursor in a first direction and
subsequently corrugating the precursor in a second direction.
11. The method of claim 10 wherein the second direction is
orthogonal to the first direction.
12. The method of claim 10 wherein the second direction is aligned
with the first direction.
13. The method of claim 10 wherein the imparting plastic
deformation step further includes the step of flattening the
precursor.
14. The method of claim 13 wherein the flattening step is performed
after at least one of the steps of corrugating the precursor in the
first direction and the second direction.
15. The method of claim 14 wherein the imparting plastic
deformation step further includes the step of corrugating in a
third direction and a fourth direction.
16. The method of claim 15 further wherein a second flattening step
is performed after at least one of the third and fourth corrugating
steps.
17. The method of claim 1 further comprising, after the step of
imparting plastic deformation, the step of net shaping the
nano-sized grain structure sheet.
18. The method of claim 17 further comprising the step of heat
treating the net shaped part to impart creep resistance to the net
shaped part.
19. The method of claim 17 wherein the step of net shaping includes
one of stamping, drawing, deep drawing and superplastic
forming.
20. The method of claim 17 wherein the step of net shaping forms an
automotive component.
21. An apparatus for performing the method of claim 1.
22. An article formed by the method of claim 1.
23. The method of claim 1 further comprising the step of providing
the sheet form with a thickness being less than that of the
precursor.
24. The method of claim 1 wherein the metal material is a metal
alloy.
25. The method of claim 1 wherein the metal material is a magnesium
alloy.
26. The method of claim 1 wherein the metal material is one
selected from the group of aluminum alloy, zinc alloy, nickel
alloys, copper alloy, .alpha./.beta. titanium alloy, steels, duplex
.alpha./.gamma. stainless steels, .alpha./.gamma. steels,
.gamma./martensite Maraging steels and metal/ceramic particle
composites.
27. The method of claim 1 wherein the step of imparting plastic
deformation includes die pressing of the fine grain precursor.
28. The method of claim 1 wherein the step of imparting plastic
deformation includes rolling the fine grain precursor.
29. The method of claim 1 wherein sheet form is provided having a
grain structure of less than about 2 micrometers.
30. The method of claim 1 wherein the sheet form is provided having
a grain structure of less than about 1 micrometer.
31. The method of claim 1 where the step of imparting plastic
deformation is performed while the precursor is heated above
ambient.
32. The method of claim 1 wherein the step of imparting plastic
deformation imparts tensile strain and compressive strain in a
strain direction.
33. The method of claim 32 wherein the step of imparting plastic
deformation is performed by passing the precursor in a first
direction between at least one pair of deforming members having
corrugated surfaces, and wherein the first direction is orthogonal
to the strain direction.
34. The method of claim 33 wherein the compressive strain is
imparted at least in part by flattening the work piece while
constraining lengthening of the work piece in the strain
direction.
35. The method of claim 34 whereby constraining lengthening of the
work piece in the strain direction is done so as to achieve one of
decreasing, increasing or preserving the thickness of the precursor
in the sheet form.
36. An apparatus for refining grain structure and producing
ultra-fine grained metal material sheets, the apparatus comprising:
a receptacle having an inlet, a discharge outlet remote from the
inlet, and a chamber defined between the inlet and the discharge
outlet; a feeder coupled with the inlet, the feeder configured to
introduce an metal material into the chamber via the inlet; a
heating device for transferring heat to the metal material located
within the chamber such that the metal material is at a temperature
above its solidus temperature; discharge means for discharging the
metal material from the receptacle through the discharge outlet;
forming means for forming and rapidly solidifying the discharged
metal material into a fine grained precursor; plastic deformation
means for imparting tensile and compressive strain into the
precursor article, the plastic deformation means deforming the
precursor article into a corrugated work piece and including a pair
of opposing forming members having protrusions formed on a surface
thereof, the protrusions of one forming member being offset from
the protrusions of the opposing forming member; the plastic
deformation means further including flattening means for flattening
the corrugated work piece into a sheet form of the metal material
having an ultra-fine grain size.
37. The apparatus of claim 36 wherein the opposed forming members
are dies.
38. The apparatus of claim 36 wherein the opposed forming members
are rolls.
39. The apparatus of claim 36 wherein the plastic deformation means
includes means for imparting tensile strain and compressive strain
into the precursor article.
40. The apparatus of claim 39 wherein the plastic deformation means
includes means for imparting first corrugations oriented in a first
direction into the precursor article and subsequently imparting
second corrugations oriented in a second direction.
41. The apparatus of claim 40 wherein the second direction is
orthogonal to the first direction.
42. The apparatus of claim 40 wherein the second direction is
aligned with the first direction.
43. The apparatus of claim 40 wherein the plastic deformation means
is configured to impart third corrugations and fourth corrugations
into the work piece.
44. The apparatus of claim 43 wherein the third and fourth
corrugations are respectively oriented in the direction of the
first and second corrugations.
45. The apparatus of claim 44 wherein the third and fourth
corrugations are out of phase with the first and second
corrugations.
46. The apparatus of claim 45 wherein the third and fourth
corrugations are respectively 180 degrees out of phase with the
first and second corrugations.
47. The apparatus of claim 36 further comprising net shaping means
for shaping the sheet form of the metal material into a net-shaped
article.
48. The apparatus of claim 47 wherein the shaping means is one of a
drawing press and a superplastic forming machine.
49. The apparatus of claim 36 wherein the receptacle, feeder,
heating means, discharge means and forming means are part of an
injection molding machine.
50. The apparatus of claim 36 wherein the receptacle, feeder,
heating means, discharge means and forming means are part of a
semi-solid metal injection molding machine.
51. The apparatus of claim 36 wherein the plastic deformation means
imparts tensile and compressive strain in a strain direction that
is orthogonal to a direction through which the work piece is passed
through the plastic deformation means.
52. The apparatus of claim 51 wherein the flattening means imparts,
at least in part, compressive strain to the work piece.
53. The apparatus of claim 52 wherein the flattening means includes
features to control lengthening of the workpiece in the strain
direction whereby the thickness of the sheet form may be controlled
so as to be increased, decreased or the same as the thickness of
the precursor.
54. A method of forming a sheet material having a refined grained
structure, the method comprising the steps of: providing a metal
material; molding and rapidly solidifying the metal alloy to form a
fine grain precursor defining a line length; initially increasing
the line length of the precursor to form a work piece; after the
step of initially increasing the line length, decreasing the line
length of the work piece; after the step of decreasing the line
length, then increasing the line length of the work piece; and
flattening the work piece to form an ultra fine grain structured
sheet form.
55. The method of claim 54 further wherein the step of flattening
the work piece is performed before the decreasing and increasing
step and then again after the decreasing and increasing step.
56. The method of claim 54 wherein the step of flattening the work
piece is performed after the decreasing step and the subsequent
increasing step.
57. The method of claim 54 wherein the initially increasing step
introduces strain into the work piece in a first direction.
58. The method of claim 54 wherein the decreasing and increasing
step introduces strain into the work piece in a second
direction.
59. The method of claim 58 wherein the second direction is
orthogonal to the first direction.
60. The method of claim 58 wherein the second direction is
generally aligned with the first direction.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims benefit of U.S. Provisional
Application No. 60/691,747 filed on Jun. 17, 2005.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to producing net shaped
components of increased strength. More particularly, the invention
relates to producing sheet components, having micrometer sized
grain structures, that can be subsequently used in the production
of net shaped sheet components of increased strength.
[0004] 2. Related Technology
[0005] Over the last several decades, magnesium (Mg) alloy
development has been inhibited by certain barriers. While wrought
magnesium has the potential for making thinner structures,
anisotropy in mechanical properties limits the applications of Mg
alloys and their wrought products. The strength of Mg alloys is
rather low in certain directions in comparison to most widely used
structural materials, such as steels and precipitation-hardened
aluminum (Al) alloys. For example, in-plane compression, yield
strength can be only 85 MPa in basal textured Mg alloy; AZ31 B Mg
alloy sheet in the H24 temper could have a high-value normal
anisotropy parameter (R) of 3.2 in the transverse direction. A high
value of normal anisotropy in sheet is helpful for deep drawing,
but may not be suitable for other applications, particularly if
in-plane strength is also anisotropic. In fact, this base element
has not been a friendly host for extensive alloy strengthening.
[0006] The alloying elements that improve corrosion resistance and
castability, such as Al, unfortunately introduce eutectic
intermetallic phases. These envelope the primary grains in a coarse
and brittle morphology. Furthermore, it is difficult to attain
efficient age hardening by fine precipitates within the grains, as
exemplified by the case of inefficient Al additions. Elements that
promote age hardening, such as rare earth metals, are costly,
detrimental to castability and ineffective in resisting corrosion.
As a consequence of these barriers, increases in strength have been
marginal, at best, and decade-old Mg alloys, such as AZ31 and
AZ91D, still dominate the tonnage of commercial sheet and casting
markets.
[0007] Hexagonal close packed (HCP) structured Mg alloys have low
symmetry of slip systems that contributes to high anisotropy of
mechanical properties. At room temperature, "basal a" slip
{0001}1120 is predominant, while "prism a" and c+a slip are
difficult because of their significantly high critical resolved
shear stresses (CRSS), which are reported in regions of high stress
concentration such as grain boundaries and twin interfaces.
Deformation twinning is often observed in polycrystalline Mg to
compensate for the insufficiency of independent slip systems. The
most common twinning modes are {1012} and {1011} twinning which
accommodate the c-axis extension and contraction, respectively. If
profusion of "tension" and "compression" twinning occur
homogeneously, good strain hardening and large ductility could
result in titanium (Ti) and zirconium (Zr). However, in Mg,
twinning is inhomogeneous, and different modes of twinning are not
initiated simultaneously. A single twinning mode can not fully
accommodate plastic deformation. When basal slip is inhibited at
ambient temperatures, twinning deformation can be localized, which
leads to low ductility in Mg.
[0008] Two major drawbacks restrict application of wrought Mg
alloys. First the symmetry of hexagonal close packed crystal
structure has the effect of limiting the number of independent slip
systems, thus providing alloys with poor formability and ductility
near room temperature. Second, forming of Mg alloys at elevated
temperatures (>300.degree. C.), although helpful to overcome the
restriction of slip, makes oxidation problems more severe.
[0009] Another means to strengthen Mg alloy, relative to Al and
steel, is by grain refinement. By the well-established Hall-Petch
relation, strength is proportional to d.sup.-1/2, where
d.about.grain size. Whereas conventional Mg alloy sheets and
extrusions have a grain size in the range of 10 to 90 .mu.m,
reducing the grain size to about 1 .mu.m or less (thus
nanostructured and herein referred to as an "ultrafine grain size")
offers a striking opportunity to escalate the strength/density of
Mg to levels above Al and steel. An ultrafine grain size could
enable superplastic deformation to be carried out at lower
temperatures and higher strain rates. At room temperature, grain
refinement strengthens many polycrystalline metals. This is true
for cubic structured metals such as Al, copper (Cu) and iron (Fe).
However, for HCP metals, such as Mg alloys, grain refinement may
also cause texture variation, and inadequate strengthening in
certain directions.
[0010] Expensive and elaborate schemes to reach an ultrafine grain
size in Mg have been developed in various research and development
efforts. A number of known methods of grain refinement, such as
rapid solidification, vapor deposition, and powder processing are
practiced in the lab. These processes are costly, time consuming
and have not enjoyed commercial success. Several other schemes for
severe plastic deformation (SPD) have proven to be unpractical for
manufacturing larger quantities of ultra-fine grain metals.
Techniques now available for performing severe deformation of bulk
material include reciprocating extrusion, three-axis plane-strain
forging, torsion under hydrostatic pressure, and equal angular
channel extrusion or pressing (ECAP). When these processes are used
in a repeated manner, the overlap of shear zones within the bulk
material from individual steps causes extensive grain subdivision
and formation of fine grain structure. Concurrent recovery and
recyrstallization processes transform subgrains with low-angle
boundaries into high-angle boundary grains. It is generally
accepted that grain refinement leads to a decrease in strain
hardening as yield strength is increased, but the variation of
strain rate sensitivity with grain refinement in Mg alloys is not
clearly documented. This may offset any loss in ductility due to
reduced strain hardening. The combination of strain hardening and
strain rate sensitivity provides a synergistic effect of a higher
tensile elongation, even when strain hardening exponent n (=d(log
.sigma.)/d(log .epsilon.)) may be lower. Deformation by several
deformation passes through equal channels (so called ECAP) has been
practiced in the lab on Mg bars, but is not practical for Mg
sheets.
[0011] Since 1999, the University of Michigan conducted research
aimed at producing sheets or billets of ultra-fine grain size,
without using a closed shearing die as used in ECAP, but by using a
multiple corrugation and flattening (MCF) process or a sine wave
deformation process (SWP) that might be more suitable for a sheet
product. The potential of the process was demonstrated with various
aluminum alloys containing dispersoid particles. This repeated
reversed plastic deformation approach was shown to achieve very
fine grain size on the surface of the sheet, which progressively
reaches the core regions of the sheet after several repeated
passes. That research showed that changes in alloy chemistry, and
use of dispersoid particles in the alloy, can take advantage of
this simpler process to produce ultra-fine grain alloys.
Application of this and other approaches to magnesium alloys was a
subject of significant interest since these alloys possess
inherently low ductility.
[0012] Under basic research funding from the National Science
Foundation, it was previously demonstrated by the University of
Michigan that, although hexagonal close packed metals, like Mg,
have inherent problems with the breakdown of coarse grains due to
textural and twinning-related issues, it is believed that either
pure compression or a constrained SWP has the capacity to overcome
these problems under suitable temperatures and process
conditions.
[0013] Superplasticity is an attribute associated with fine-grained
alloys. This plastic-type property is utilized commercially in
automobiles and aircrafts to form complex net shapes in titanium
(Ti) and Al. To date, Mg alloys have not enjoyed this advantageous
processing in commerce. First, Mg alloy castings do not have the
prerequisite grain boundary crystal structure and, secondly,
wrought Mg sheets have been too coarse grained and/or too textured
for superplastic forming.
[0014] Turning to more definitive discussion of nanotechnology,
nano-size strengthening phases of about 100 nanometers are
desirable within the grains. This is another strengthening
mechanism, heretofore not available in weakly alloyed AZ31 sheets.
However, construction and assembly of such a microstructure for
bulk structural parts, ab-initio from nano-powders, is a very
costly and laborious. Also, there are safety and health concerns
for handling such fine particles in the workplace. It seems to be
safer and more practical to generate such nano-strengthening
particles in-situ during processing of the already assembled bulk
component.
[0015] Grain size has a major effect on the formability of Mg alloy
sheets. Currently, commercial wrought Mg alloy sheet is available
only in low strength AZ31 alloy. It is fabricated from direct cast
(DC) slabs (0.3 m thick) having a grain size of 200-1000 .mu.m.
Twin roll casting (TRC), a prototype process, is offered at 2 to 5
mm thicknesses with 60 to 2000 .mu.m grain sizes and is currently
only capable of 432 mm wide sheets. Fabrication from DC or TRC
promotes strong texture because of the limited slip systems and
twinning occurring in Mg alloys with such large grain sizes.
Extrusions formed from such a base source are also textured to the
extent that strength is 50% and toughness is 72% in one direction
as compared to the cross direction. The grain boundary structure in
conventionally prepared Mg alloy is not favorable to complex
deformation without premature fracture, unless an elevated forming
temperature is used. The pressing and deep drawing of 3-D shapes is
limited by the texture and the inherent non-uniform deformation
that results from twinning. Although twinning in some directions of
the sheet causes increased elongation during tensile testing,
twinning is an impediment to the formation of complex parts due to
the anisotropy it produces in coarse grain Mg alloy, resulting in
anomalies in work hardening and non-uniform deformation. Further,
the modeling of forming processes and performance in the dies is
not reliable with such non-uniformity in structure. Also, the
coarse surface finish of present coarse grain Mg alloys poses a
challenge to their acceptance as automotive sheet parts.
[0016] To minimize the adverse effects of coarse grains and
twinning, conventional wrought alloy processes use multiple rolling
and annealing operations until the grain size becomes finer. The
TRC product is typically too thin to refine the grain size below 7
.mu.m by such hot processing. The TRC structure also suffers from
centerline porosity. Continuous cast Mg alloy may have promise, but
currently this technology is not fully developed and many
individual pieces of technologies are required for its full
implementation, the scope of which is incompatible with small
business operations and may not have the flexibility offered by the
process of the present invention. Further, the slag and dross of
known processes would be conducive to the attacking of the
refractories by the processed material; SF.sub.6 gas (a global
warming gas) may be a manufacturing by-product; and trapped
inclusions may result from any necessary flux. The many stages
involved in breaking down large-grained conventional sheet
precursors to produce the sheet form cause current wrought Mg
alloys to be expensive, on the order of $5.00 to $10.00/lb.
[0017] As seen from the above, there exists a need for an apparatus
and process that can be carried out in a rapid and automated manner
so as to change alloy composition and grain structure, thereby
allowing such processed alloys to be subsequently worked into
net-shaped sheet products.
SUMMARY
[0018] In achieving the above object, the inventors have discovered
a practical new process and apparatus to generate inexpensive ultra
fine grain structured sheets, where grain sizes of less than or
equal to about 2 .mu.m are achieved, which can be subsequently
deformed via superplastic forming processes to form net shaped,
sheet formed articles. Various metals and alloys can be employed
with the present invention, including, but not limited to, Mg, Al,
zinc (Zn), nickel (Ni), copper (Cu), .alpha./.beta. Ti, steels,
duplex .alpha./.gamma. stainless steels, .alpha./.gamma. steels,
.gamma./martensite Maraging steels and metal/ceramic particle
composites.
[0019] The present process involves the sine-wave deformation
processing (SWP) of fine grain structured sheets initially formed
from various rapid solidification molding methods that can produce
an ultra fine grain precursor, including injection molding and
variations on injection molding, extrusion molding and twin roll
casting. Thereafter, the final net shaping of parts can be
accomplished by superplastic forming, drawing or stamping, etc.
Thus, the present invention provides for the initial formation of a
fine grained precursor having a grain size of less than about 10
.mu.m. Thereafter, the fine grained precursor is subjected to SWP,
which breaks down the microstructure of the precursor and produces
new grain boundaries. The resulting sheet has an ultra fine grain
structure lending itself to final net shaping by superplastic
forming processes. Accordingly, in one aspect the present invention
is a method of forming a sheet material having a refined grained
structure, the method comprising the steps of: providing a metal
material; molding and rapidly solidifying the metal alloy to form a
fine grain precursor; and imparting plastic deformation to the fine
grain precursor by a combination of alternating tensile strain and
compressive strain to form an ultra fine grain structured sheet
form.
[0020] In another aspect, the step of molding and solidifying
develops a mutliphased microstructure in the fine grained
precursor.
[0021] In a further aspect, the multiphased microstructure includes
pinning particles that minimize grain growth.
[0022] In yet another aspect, the step of imparting plastic
deformation includes the step of storing dislocations in the
microstructure.
[0023] It is also an aspect that step of imparting plastic
deformation includes the step of causing the formation of new grain
boundaries having high misorientation suitable for warm forming or
superplastic forming.
[0024] Still another aspect is that the molding step and the
imparting plastic deformation step are performed in an integrated
apparatus.
[0025] A further aspect is that the molding step and the imparting
plastic deformation step are performed by separate machines.
[0026] In another aspect, the molding step includes semisolid metal
injection molding of the metal material.
[0027] In a further aspect, the molding step includes one of
extruding of the metal material and twin roll casting of the metal
material.
[0028] In yet another aspect, the imparting plastic deformation
step includes corrugating the precursor in a first direction and
subsequently corrugating the precursor in a second direction.
[0029] It is also an aspect the second direction is orthogonal to
the first direction.
[0030] Still another aspect is that the second direction is aligned
with the first direction.
[0031] A further aspect is that the imparting plastic deformation
step further includes the step of flattening the precursor.
[0032] In another aspect, the flattening step is performed after at
least one of the steps of corrugating the precursor in the first
direction and the second direction.
[0033] In a further aspect, the imparting plastic deformation step
further includes the step of corrugating in a third direction and a
fourth direction.
[0034] In yet another aspect, a second flattening step is performed
after at least one of the third and fourth corrugating steps.
[0035] In an additional aspect, after the step of imparting plastic
deformation, the step of net shaping the nano-sized grain structure
sheet.
[0036] In another aspect, the invention further includes the step
of heat treating the net shaped part to impart creep resistance to
the net shaped part.
[0037] In still another aspect, the step of net shaping includes
one of stamping, drawing, deep drawing and superplastic
forming.
[0038] In another aspect, the step of net shaping forms an
automotive component.
[0039] In another aspect, the invention includes an apparatus for
performing the above mentioned method.
[0040] In yet another aspect, the invention includes an article
formed by the above mentioned method.
[0041] In another aspect, the invention further includes the step
of providing the sheet form with a thickness being less than that
of the precursor.
[0042] In another aspect, the metal material is a metal alloy.
[0043] In an additional aspect, the metal material is a magnesium
alloy.
[0044] In another aspect, the metal material is one selected from
the group of aluminum alloy, zinc alloy, nickel alloys, copper
alloy, .alpha./.beta. titanium alloy, steels, duplex
.alpha./.gamma. stainless steels, .alpha./.gamma. steels,
.gamma./martensite Maraging steels and metal/ceramic particle
composites.
[0045] In further another aspect, the step of imparting plastic
deformation includes die pressing of the fine grain precursor.
[0046] In another aspect, the step of imparting plastic deformation
includes rolling the fine grain precursor.
[0047] In another aspect, the sheet form is provided having a grain
structure of less than about 2 micrometers.
[0048] In still another aspect, the sheet form is provided having a
grain structure of less than about 1 micrometer.
[0049] In an additional aspect, the step of imparting plastic
deformation is performed while the precursor is heated above
ambient.
[0050] In one aspect, the step of imparting plastic deformation
imparts tensile strain and compressive strain in a strain
direction.
[0051] In another aspect, the step of imparting plastic deformation
is performed by passing the precursor in a first direction between
at least one pair of deforming members having corrugated surfaces,
and wherein the first direction is orthogonal to the strain
direction.
[0052] In yet anther aspect, the compressive strain is imparted at
least in part by flattening the work piece while constraining
lengthening of the work piece in the strain direction.
[0053] In still another aspect, constraining lengthening of the
work piece in the strain direction is done so as to achieve one of
decreasing, increasing or preserving the thickness of the precursor
in the sheet form
[0054] In another aspect, the invention is an apparatus for
refining grain structure and producing ultra-fine grained metal
material sheets, the apparatus comprising: a receptacle having an
inlet, a discharge outlet remote from the inlet, and a chamber
defined between the inlet and the discharge outlet; a feeder
coupled with the inlet, the feeder configured to introduce an metal
material into the chamber via the inlet; a heating device for
transferring heat to the metal material located within the chamber
such that the metal material is at a temperature above its solidus
temperature; discharge means for discharging the metal material
from the receptacle through the discharge outlet; forming means for
forming and rapidly solidifying the discharged metal material into
a fine grained precursor; plastic deformation means for imparting
strain into the precursor article and deforming the precursor
article into a corrugated work piece, the plastic deformation means
including a pair of opposing forming members having protrusions
formed on a surface thereof, the protrusions of one forming member
being offset from the protrusions of the opposing forming member;
and flattening means for flattening the corrugated work piece into
a sheet form of the metal material having an ultra-fine grain
size.
[0055] In another aspect, the opposed forming members are pressing
dies or rolls.
[0056] In further another aspect, the plastic deformation means
includes means for imparting tensile strain and compressive strain
into the precursor article.
[0057] In another aspect, the plastic deformation means includes
means for imparting first corrugations oriented in a first
direction into the precursor article and subsequently imparting
second corrugations oriented in a second direction.
[0058] In another aspect, the second direction is orthogonal to the
first direction.
[0059] In still another aspect, the second direction is aligned
with the first direction.
[0060] In another aspect, the plastic deformation means is
configured to impart third corrugations and fourth corrugations
into the work piece.
[0061] In an additional aspect, the third and fourth corrugations
are respectively oriented in the direction of the first and second
corrugations.
[0062] In yet another aspect, the third and fourth corrugations are
out of phase with the first and second corrugations.
[0063] In another aspect, the third and fourth corrugations are
respectively 180 degrees out of phase with the first and second
corrugations.
[0064] In another aspect, the invention further includes net
shaping means for shaping the sheet form of the metal material into
a net-shaped article.
[0065] In further aspect, the shaping means is one of a drawing
press and a superplastic forming machine.
[0066] In another aspect, the receptacle, feeder, heating means,
discharge means and forming means are part of an injection molding
machine.
[0067] In an additional aspect, the receptacle, feeder, heating
means, discharge means and forming means are part of a semi-solid
metal injection molding machine.
[0068] In another aspect, the invention is a method of forming a
sheet material having a refined grained structure, the method
comprising the steps of: providing a metal material; molding and
rapidly solidifying the metal alloy to form a fine grain precursor
defining a line length; initially increasing the line length of the
precursor to form a work piece; decreasing the line length of the
work piece and then increasing the line length of the work piece;
and flattening the work piece to form an ultra fine grain
structured sheet form.
[0069] In still another aspect, the step of flattening the work
piece is performed before the decreasing and increasing step and
then again after the decreasing and increasing step.
[0070] In another aspect, the step of flattening the work piece is
performed after the decreasing and increasing step.
[0071] In an additional aspect, the initially increasing step
introduces strain into the work piece in a first direction.
[0072] In further aspect, the decreasing and increasing step
introduces strain into the work piece in a second direction.
[0073] In another aspect, the second direction is orthogonal to the
first direction.
[0074] In yet another aspect, the second direction is generally
aligned with the first direction.
[0075] It is also an aspect of the invention that the plastic
deformation means imparts tensile and compressive strain in a
strain direction that is orthogonal to a direction through which
the work piece is passed through the plastic deformation means.
[0076] It is a further aspect that the flattening means imparts, at
least in part, compressive strain to the work piece.
[0077] It is yet another aspect that the flattening means includes
features to control lengthening of the workpiece in the strain
direction whereby the thickness of the sheet form may be controlled
so as to be increased, decreased or the same as the thickness of
the precursor.
[0078] Further features and advantages of this invention will
become readily apparent to persons skilled in the art after a
review of the following description, with reference to the drawings
and claims that are appended to and form a part of this
specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0079] FIG. 1 is schematic illustration of a manufacturing cell
embodying the principles of the present invention;
[0080] FIG. 2A is a perspective view of the longitudinal roll dies
shown in FIG. 1 and which may be utilized with the present
invention;
[0081] FIG. 2B is a perspective view of the transverse roll dies
seen in FIG. 1 which may be used with the present invention;
[0082] FIG. 2C is perspective view of flattening roll dies as seen
in FIG. 1 and used in connection with the present invention;
[0083] FIG. 2D is perspective view of a pair of pressing dies that
may be utilized as an alternative to roll dies in accordance
another embodiment of the present invention;
[0084] FIG. 3 is a flowchart of one possible process in accordance
with the present invention;
[0085] FIG. 4 is a flowchart of another possible process
incorporating the principles of the present invention;
[0086] FIG. 5 is a diagrammatic illustration of the present
invention incorporating an extrusion device;
[0087] FIG. 6 is a schematic illustration of a twin roll casting
device incorporated with the present invention;
[0088] FIG. 7 is a graphical comparison of the effect of grain size
(d) on hardness (Hr) for SWP AZ91D and AZ31B;
[0089] FIG. 8 is a representation of the results of a superplastic
bulge test (processed at 280.degree. C. and 200 psi) as a function
of initial grain size; and
[0090] FIGS. 9A, 9B, 9C and 9D are schematic illustrations of a
precursor undergoing SWP according to another embodiment of the the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0091] According to one aspect and embodiment of the present
invention, a fine grained precursor is formed by the injection
molding of metal, such as by the Thixomolding.TM. process of
Thixomat, Inc., Ann Arbor, Mich. According to this process, melt
temperatures can be lowered to near liquidus, some 80 to
100.degree. C. lower than in DC or TRC. These lower temperatures
assist in faster cooling to nucleate finer grains upon
solidification. As injection molded, Thixomolded.TM. Mg alloys are
isotropic with 4 to 5 .mu.m grain size a phase. Through the use of
multiple feeding ports, the rapid injection molding of large sheet
bars is possible. Suitable sheet bar would be readily molded in
existing commercial Thixomolding machines, of sizes up to 1000
tons, with sheet dimensions of 20.times.400.times.400 mm.
[0092] Table 1 presents various production methods for a precursor
work piece, such as sheet bar, as well as for a range of grain
sizes resulting from that production method including the method of
the present invention. TABLE-US-00001 TABLE 1 Effect of Process on
Grain Size of Mg Sheet Process Condition Grain Size (.mu.m) Direct
Cast As Cast Billet (300 mm) 200 Direct Cast Extruded 8-90
Twin-roll Cast As Cast (2-5 mm) 60-2000 Twin-roll Cast Hot Rolled
7-10 Injection Molded As Molded 4-5 Injection Molded + SWP As SWP
.ltoreq.1
[0093] Referring to FIG. 1, this figure schematically illustrates
an apparatus, generally designated at 8, embodying the principles
of the present invention. The apparatus 8 includes a molding
machine 10 for the metal injection molding of sheet bar. As seen in
FIG. 1, the construction of the molding machine 10 is, in some
respects, similar to that of a plastic injection molding machine.
The machine 10 is fed with feedstock 11 via a hopper 12 into a
heated, reciprocating screw injection system 14 which maintains the
feedstock under a protective atmosphere such as argon. More
particularly, the feedstock is received into a barrel 15 via an
inlet 16 located at one end of the barrel 15. Within the barrel 15,
the feedstock is moved forward by the rotating motion of a screw
18. As the feedstock is moved forward by the screw 18, it is also
heated by heaters 20 (which may be a resistance, induction or other
type of heater) while being stirred and sheared by the action of
the screw 18. This heating and shearing is done to bring the
feedstock material into a injectable state. This injectable
material passes through a non-return valve 22 and into an
accumulation zone 24, located within the barrel 15 beyond the
forward end of the screw 18. Upon accumulation of the needed amount
of injectable material in the accumulation zone 24, the injection
portion of the cycle is initiated by advancing the screw 18 with a
hydraulic or other actuator 25. Advancement of the screw 18 causes
the material in the accumulation chamber 24 to be ejected through a
nozzle 26 into a mold 28 filling the mold cavity defined thereby
and forming a precursor work piece such as sheet bar 30. This
initial formation of the precursor allows developing of a
multiphase microstructure with pinning particles or phases to pin
grain boundaries to minimize grain growth.
[0094] In one preferred embodiment, the metallurgical process of
the machine 10 results in the processing of the particulate
feedstock into a solid plus liquid phase prior to its injection
into the mold 28. Various versions of this basic process are known
and two such versions are disclosed in U.S. Pat. Nos. 4,694,881 and
4,694,882, which are herein incorporated by reference. The process
generally involves the shearing of the semisolid metal so as to
inhibit the growth of dendritic solids and to produce non-dendritic
solids within a slurry having improved molding characteristics
which result, in part, from its thixotropic properties. (A
semisolid non-dendritic material exhibits a viscosity that is
proportional to the applied shear rate and which is lower than that
of the same alloy when in a dendritic state). Variations on this
process of forming the sheet bar 30 may include providing the alloy
material initially in a form other than a particulate; heating the
alloy material to an all liquid phase and subsequently cooling into
the solid plus liquid phase; utilize twin screws for processing the
alloy; employing separate vessels for processing of the alloy and
injecting of the alloy; utilizing gravity or other mechanisms to
advance the alloy through the barrel to the accumulation zone;
alternate feeding mechanism, including electromagnetic; and other
variations on the process.
[0095] In another preferred embodiment, metallurgical process of
the machine 10 results in the processing of the particulate
feedstock into an all liquid phase that is injected into the mold
28 and rapidly solidified.
[0096] Once the fine grained sheet bar 30 is formed, it is
subjected to SWP. Generally, SWP involves the imparting of plastic
deformation by a combination of alternating tensile and compressive
strains or deformations. This second step permits storage of
dislocations within the microstructure, which leads to the
formation of new grain boundaries with high misorientation suitable
for subsequent warm forming or superplastic forming.
[0097] In one implementation of the SWP process, the precursor is
subjected repeated shaping of the material between a pair of
corresponding members having corrugated or sine-wave shaped forming
surfaces. The shape of the forming surfaces impart large strain,
breakdown the cast microstructure and produce new grain boundaries
in the precursor. Beginning with sheet bar 30, initially formed so
as to have a fine grain structure of 10 .mu.m or less, this
precursor work piece is then shaped, with or without lateral
constraint, between two members having corresponding corrugated
forming surfaces, in what is essentially a plane-strain
stretch-bend operation. After the first shaping, the work piece is
again shaped, During this second shaping, however, the corrugations
are preferably, but not necessarity, oriented in a direction
different from the corrugations of the first shaping. An orthogonal
orientation for the second shaping is believed to produce the best
end results. Preferably, the two shaping steps are then repeated
with the corrugations in these third and fourth steps being the
inverse of those seen in the first two shaping steps. By the term
inverse, what is meant is that the ridges and valleys of the third
corrugation are reversed or out of phase from the ridges and
grooves of the first corrugations. Thus, these subsequent shaping
cause a reverse deformation (pushing in the opposite direction) of
ridges resulting after the first two shapings. After all four
shaping steps, and additional shaping steps if desired, the work
piece is preferably flattened to remove any waviness to the shape.
As further discussed below, the work piece may alternatively be
flattened between each shaping step or after the first two shaping
steps.
[0098] The alternating tensile and compressive deformations are
imparted by creating a general sine wave shape in the precursor.
This shaping increases the line length (the length of the
centerline of the precursor). Following this with a reverse sine
wave shaping of the work piece results in the line length initially
being shortened and then again lengthened as the raised ridges of
the shape are converted in recessed valleys. When undergoing
flattening, the line length is thus shortened. Accordingly,
increasing the line length introduces tensile strain into the work
piece and reducing the line length introduces compressive
strain.
[0099] Preferrably, SWP is conducted at a warm temperature and the
deformation temperature of the material is progressively lowered
after each pass, for example, starting at 250.degree. C. and
decreasing to 170.degree. C. for the final flattening step. This
can be achieved in several ways, including providing heated shaping
members or rolls, as described below.
[0100] After four alternate shaping steps, the strain in the
mid-plan has gone from tension to compression to tension again in
both the 0.degree. and 90.degree. directions. With these successive
shapings, all parts of the sheet bar material are deformed in a
manner of "kneading", by incorporating reversed plane strain
bending and plane strain stretching in two orthogonal directions of
the plate. The repeated deformation by the process causes
accumulation of large plastic strain and breaks up the original
grain structure within the work piece. Grain refinement occurs
initially heterogeneously, but eventually homogeneously, over a
large area. While four shapings are discussed in one of the
preferred embodiments, more or less shapings may be possible to
achieve the desired results.
[0101] Various schemes can be envisioned for deforming the sheet
bar 30 during SWP. SWP can be achieved by passing the sheet bar 30
successively through a series of rolls or by pressing the sheet bar
successively between a pair of opposing pressing dies, either of
which may be heated. Also, SWP may be performed separately (at a
remote location) from the formation of the sheet bar 30 or may be
integrated directly into a processing cell whereby the apparatus 8
is provided with a transfer mechanism (which may be any known
variety and which is represented by line 29) to transfer the sheet
bar 30 from the mold 28 to a rolling or pressing mill 31. As seen
in FIG. 1, the apparatus 8 includes a rolling mill 31, having a
series of roll sets, integrated with the molding machine 10. The
rolls of the roll sets may be impressed toward each other by backup
rolls 33 (shown in phantom) as is commonly known.
[0102] In the illustrated rolling mill 31, the sheet bar is passed
through a first set 32 of opposed corrugated rolls 34. The surfaces
of the rolls 34 are each provided with corrugations 36 extending
circumferentially about the rolls 34. The corrugations 36 of each
rolls 34 generally correspond with respect to one another such that
a ridge on one of the rolls 34 is received in a valley of the
opposing rolls 34. As the sheet bar 30 passes through the first set
32 of rolls 34 a lengthwise corrugation, parallel to the direction
of travel of the sheet bar 30, is imparted into the sheet bar 30.
This results in a sine wave shape being imparted to the work piece
that is oriented in a direction orthogonal to the direction in
which the work piece is passed through the rolling mill 31.
Accordingly, the induced strains, tensile and thereafter
compressive, will be generally in the direction of the sine wave
shape itself. In order to constrain lateral expansion of the sheet
bar 30, one of the rolls, the lower roll 34 in FIG. 1, may be
provided with raised lands 38 on the opposing ends of the roll 34.
The lateral most corrugations of the upper roll 34 fit within and
extend below the uppermost surface of the lands 38. As a result,
the sheet bar 30 is constrained from expanding laterally beyond the
lands 38 of the rolls 34. These rollers, 34 are separately
illustrated in FIG. 2A apart from the subsequent rollers.
Alternately, lateral expansion may be unconstrained.
[0103] Having been corrugated or worked by the first set 32 of
rolls 34, the worked sheet bar or work piece is passed to a second
set 40 of rolls 42. Upon encountering this second set 40 of rolls
42, the work piece encounters corrugations 44 that are oriented
orthogonally, 90 degrees from the corrugations 36 of the first set
32 of rolls 34. As such, the corrugations 44 are oriented axially
with respect to the rolls 42 and transverse with regard to the
direction of travel of the sheet bar 30. As with the prior set 32
of rolls 34, the corrugations 44 of the second set 40 of rolls 42
are provided such that the ridge of a corrugation on the upper roll
42 is received within the valley of a corrugation 44 of the lower
roll 42. Raised lands 46 may be formed on the lower roll 42 so as
to define constraints and prevent lateral lengthening/expansion of
the work piece as it is passed through the second set 40 of rolls
42. The rollers 42 of the second set 40 are separately illustrated
in FIG. 2B and could alternatively be provided such that lateral
expansion is unconstrained.
[0104] From the second set 40 of rollers 42, the worked sheet bar
is passed in the illustrated rolling mill 31 between a third set 48
of rolls 50 designed to flatten the worked sheet bar. To achieve
this, the rolls 50 are provided with smooth surfaces 52 that engage
and compress the worked sheet bar as it passes between the rolls
50. As the work piece is flattened, compressive strain in imparted
to the work piece. Similar to the prior two sets 32, 42 of rolls
34, 42, the lower roll 50 of the third set 48 of rollers includes
raised lands 54 to constrain and inhibit lateral
lengthening/expansion of the worked sheet bar as it passes between
the rolls 50. By adjusting the lateral position and constraint
provided by the lands 54 of the rolls 50, the thickness of the
resulting sheetstock material 78 can be controlled so as to be
decreased, increased or the same as the original thickness of the
sheet bar 30, all the while continuing to accumulate plastic
deformation to the sheetstock material 78. The rolls 50 of the
third set 48 are separately illustrated in FIG. 2C.
[0105] Once exiting the third set 48 of rolls 50, the work piece
may again be subjected to corrugation and the process of passing
the work piece through the three sets of rollers is repeated, as
suggested by dashed line 56.
[0106] As an alternative scheme for deforming the sheet bar 30,
pressing plates 58 (one set 59 of which is representatively
illustrated in FIG. 2D) may be used in place of the sets of rolls
32, 40, 48. As seen in FIG. 2D, the plates 58 are provided with
cooperating corrugations 60 in which the ridge of one corrugation
interfits with the valley of the opposing corrugation. Similarly, a
raised perimeter or land 62 is provided about the periphery of one
of the plates, herein the lower plate 60, so as to laterally
constrain the sheet bar 30 as it is pressed therebetween.
[0107] As thus far described, SWP occurs generally according to the
process illustrated by the flowchart of FIG. 3. As shown therein,
SWP starts at box 66 wherein a sheet bar 30 is received and
subjected to corrugating in a lengthwise or parallel direction in
box 68. After lengthwise corrugating of the sheet bar 30, the work
piece undergoes transverse corrugation in box 70 and subsequently
is flattened as indicated in box 72. After being flattened in box
72, the lengthwise and transverse corrugating of the work piece may
be repeated as indicated by line 74. Optionally, as indicated by
phantom line 76, the work piece can undergo subsequent lengthwise
and transverse corrugation prior to being flattened in box 72.
However, it is believed to be preferable that flattening according
to box 72 occurs prior to subsequent corrugation of the work piece.
After proceeding through the corrugation process wherein both
lengthwise and transverse corrugation occurs twice (thus
corrugating of the work piece four times) the work piece is finally
flattened in box 72 and flat sheetstock material 78 is outputted
and the process ends in box 80. While not illustrated, it is
contemplated that the transverse corrugation of box 70 can be
replaced with an additional lengthwise corrugation, aligned with or
off set from the initial lengthwise corrugation.
[0108] While the above process has been described whereby a
lengthwise corrugation of the sheet bar occurs prior to a
transverse corrugation, it should be apparent that the two sets 32,
40 of rolls 34, 42 can be alternated in their positions such that
the transverse corrugations 44 of rolls 42 first encounter the
sheet bar 30 and the lengthwise corrugations 36 of rolls 34
encounter the work piece after initial corrugation. This order of
the rolls may be advantageous in that the orientation of the
corrugations 44 would allow for a "grabbing" of the sheet bar 30
possibly facilitating entrance of the sheet bar 30 into the initial
rolls, as well as subsequent rolls.
[0109] Two alternative methods for SWP are illustrated by the
flowchart of FIG. 4. According to this process, after each
corrugation of the work piece, a flattening of the work piece
occurs. Accordingly, SWP begins in box 82 where sheet bar 30 is
transferred to the first set 32 of rolls 34 where it undergoes
lengthwise corrugation in box 84. After lengthwise corrugation, an
additional set of rolls, similar to the flat rolls 50 of the
previously mentioned third set 48 are provided so as to flatten the
work piece in box 86. The worked and flattened work piece is
thereafter transferred to another set of rolls where transverse
corrugation occurs in box 88. After transverse corrugation of the
work piece, a set of rolls again flatten the work piece in box 90.
At this stage the work piece is transferred back to the first set
of rollers, according to line 92, where lengthwise corrugation is
again performed in box 84. The work piece then proceeds through
flattening (box 86), transverse corrugation (box 88), flattening
(box 90) and the formed flat sheetstock material 78 is produced at
the end of SWP as indicated by box 94.
[0110] As mentioned above, the alternating tensile and compressive
deformations are imparted by creating a generally sine wave shape
in the precursor 30. FIGS. 9A-9D illustrate the concept of line
lengthening and shortening, as well as lateral constraint, in
another embodiment of the invention.
[0111] As seen in FIG. 9A, the precursor 30 undergoes plastic
deformation in a set of non-constraining rolls 334 that are
generally analogous to the rolls 34 of FIG. 1. The rolls 334
include mating or corresponding corrugations 336 that extend
circumferentially around the rolls 334. As a result, a generally
sine wave shape is imparted into the worked precursor (the work
piece), generally orthogonally to the direction of rolling. In that
the rolls 334 are non-constraining, the work piece is lengthened
along its centerline visa vi the sine wave shape. An initial
rolling or corrugating of the precursor 30 thus lengthens and
imparts tensile strain to the work piece. Subsequently, as seen in
FIG. 9B, the work piece is at least partially flattened (imparting
compressive strain) by the flat surfaces 352 of rolls 350, which
are generally analogous to the rolls 50 of FIG. 1. Rolls 350
laterally constrain, via lands 354, the work piece as it is
flattened and as such the line length is reduced during flattening.
Next, as seen in FIG. 9C, the work piece under goes a corrugation
between another set of rolls 360 having corrugations 362 that are
circumferential or parallel to the rolling direction of the rolls
360. This corrugation is a reverse corrugation, however, in that
the corrugations 362 of this set of rolls 360 are reversed from
corrugations of corrugated roll 334. In other words, the peaks and
valleys of these rolls are generally oppositely oriented relative
to those of the prior corrugated rolls 332. If the work piece has
not been completely flattened by flat rolls 350, this set of
corrugated rolls 362 will first compress and then stretch the work
piece as previously described elsewhere in this specification,
resulting in the overall lengthening of the line length of the work
piece. Again, the corrugation may be done without lateral
constraint by the rolls 362. Finally, as seen in FIG. 9D, the work
piece is compressed and the sine wave shape completely flatten
between a set of rolls 372. (Obviously, intermediate rolls 362 and
the final set of rolls 372, the work piece may undergo any number
of corrugations cycles (where the strain is aligned, orthogonal to
or otherwise oriented with respect to the previously induced
strain), or other processing steps, imparting tensile and
compressive strains so as to accumulate deformation in the work
piece.) At rolls 372, the work piece is laterally constrained by
lands 374 so that the final sheet form 378 is outputted for further
processing. By controlling the lateral contraint of the workpiece,
it is possible to produce a resultant sheet form that has a
thickness that is decreased, increased or the same as the thickness
of the precursor.
[0112] It is noted that the rolls of FIGS. 9A-9D are illustrated in
a side by side positioning. As such, the construction is
representative of a reverse rolling mill. In such a reverse rolling
mill, the work piece is provided through one roll in one
longitudinal direction and through a subsequent roll in a generally
reversed longitudinal direction. Reverse rolling mills themselves
are well known in the art and further elaboration on the
construction is therefore warranted herein.
[0113] As previously mentioned, various schemes for the
manufacturing of the initial precursor, the sheet bar 30 discussed
above, are believed possible if proper and precise control of the
manufacturing process and rapid solidification thereof is done.
FIGS. 5 and 6 schematically illustrate two additional manufacturing
schemes wherein the injection molding machine 10 of the first
embodiment is alternately replaced with an extrusion machine 110
(in FIG. 5) and a twin roll casting machine 210 (in FIG. 6).
[0114] Referring now to FIG. 5, the extrusion machine 110 includes
a barrel 112 within which is located a screw 114. In that the other
components of an extrusion machine are well known to those skilled
in the art, additional discussion of the extrusion machine 110 is
not provided herein. Material is extruded from the extrusion
machine 110 and rapidly solidified between a pair of molds 116 such
that a continuous sheet of solid material is transferred from the
extrusion machine to the rolling mill 31. By precisely controlling
the process of the extrusion machine, it is believed that the
required fine grained microstructure can be achieved in a
continuous sheet, which operates as the precursor material into the
rolling mill 31 in accordance with the present invention. The
rolling mill 31 illustrated in FIG. 5 is similar to the rolling
mill 31 discussed in connection with the prior embodiment.
Accordingly, reference is hereby made thereto and further
discussion is not required.
[0115] As seen in FIG. 6, the twin roll casting machine 210
includes a pair of counter-rotating rolls 212 which receive the
processed material 214 from a processing vessel 216. Between the
rolls 212, the material 214 is rapidly solidified into a precursor
sheet form 218 of a first thickness 220. By precisely controlling
the twin roll casting machine 210, it is anticipated that the fine
grain microstructure required according to the present invention
can be achieved. Accordingly, by transferring the precursor
material 214 to a rolling mill 231 similar to that previously
described, the material 214 can be reduced to a final sheet form
222 having the desired reduced thickness 224. Since the rolling
mill 31 of FIG. 6 is substantially of the same construction as that
previously described in connection with FIG. 1, reference thereto
is herein made.
[0116] With a 400.times.400.times.20 mm sheet bar 30 as the
precursor, the above described SWP process can reduce the thickness
of the sheet to about 2 mm, wherein the final sheet dimensions
could be 1250.times.1250 mm. With thinner starting materials, such
as 6.35 mm hot rolled plate, it is anticipated that the thickness
can be reduced to about 1 mm. Alternatively, the SWP process can
produce a sheet maintain the original starting thickness of the
precursor or can actually produce a thickened sheet. The latter is
achieved by further constraining lateral expansion of work piece,
after the work piece has been shortened via a corrugating step, to
a dimension that is less than the starting dimension of the
precursor.
[0117] When an integrated automated manufacturing cell, such as one
of those previously described, combines the rapid solidification of
metal injection molding with SWP as part of the same manufacturing
cycle, the rate of production in one machine is anticipated to be
about 1 sheet bar per 20 seconds.
[0118] As would be surmised from the preceding discussion of the
invention, the as-molded grain size and a content of an injection
molded metal sheet bar is a favorable starting point to attaining
sub-micron grain size and low-anisotropy in the subsequently SWP
sheet. It appears that SWP, with its vigorous thermomechanical
working, subdivides intermetallic particles into nano-sizes, and,
probably, encourages partial solution and more homogeneous
reprecipitation of fine arrays within the grains. Some sub-divided
residual .beta. phase could serve to pin grain boundaries during
dynamic recrystallization and heat treatment. The subdividing of
this inherently coarse .beta. phase is beneficial to the ductility
of Mg alloys. The same role is anticipated as being possible with
two phase alloys, such as .alpha./.beta. Ti, .alpha./.gamma.
stainless steels, .gamma./martensite Maraging steels and cementite
steels.
[0119] The aforementioned .beta. phase effect is but one aspect of
the new opportunities to redesign Mg for this new process. The
literature is replete with new Mg alloying discoveries that have
yet to be applied to a low cost sheet form. These alloying
additions are easily reduced to sheet form by the present
invention, especially utilizing "blending" techniques. Such
alloying additions as Ca, Sr, Y, Zr and Zn--Y can boost the modest
strength of the commercial sheet alloy AZ31. Additionally, the
large melts and alloy cross contamination, which are inherent in DC
and TRC, can be avoided by using the above mentioned injection
molding SWP process. Purging of the previous alloy and addition of
granules of new blends can be accomplished in minutes in an
injection molding machine, without the wasted crucible charges,
slag and dross typically associated with DC or TRC operations.
[0120] Ductility during warm temperature stamping (and superplastic
forming) of metals is enhanced by the presence of many grain
boundaries, but grain boundaries developed from current casting
processes are unsuitable for forming applications because they do
not permit rolling or sliding between grains. Grain boundary
character has a major effect on the phenomena of sliding and
shearing properties of grain boundaries during deformation. Even at
modestly elevated temperatures (150-200.degree. C.), Mg alloys can
be formed easily by warm forming processes, provided they have a
fine grain structure (about 1-3 .mu.m) and favorable grain
boundaries produced by deformation processing. While forming of an
alloy at room temperature is preferred, 150-200.degree. C.
temperatures are not unusual for inexpensive forming applications
(plastics are often formed at such temperatures). Unlike plastics
however, Mg parts can be heat treated to grow larger grain size and
become creep resistant, or can be alloyed appropriately to make
them creep resistant. Low temperature forming can however keep
energy usage low during forming and avoid undesirable oxidation
encountered during the superplastic forming process.
[0121] The rapid solidification during the injection molding
process provides a fine grain structure that does not exhibit
twinning during subsequent deformation. However, grain boundaries
created from the liquid state are crystallographically related, and
may possess "special" boundaries that do not permit grain boundary
sliding. Special boundaries may have high misorientation angles,
but they could have a significant fraction of coincident lattice
sites (CSL) and low grain boundary energies to make sliding
difficult. While the strain contributed by grain boundary sliding
is not large during warm forming, if it is capable of providing
accommodation locally, it prevents fracture of the material along
grain boundaries. Thus, the boundaries required for enhanced
formability must not be those produced by the casting process, but
those generated by the plastic working process. The plastic working
generates additional dislocations near the grain boundaries and
renders then into configurations of higher disorder or higher
energy, suitable for enhanced formability.
[0122] The above understanding, although derived from basic
principles of materials science, has been repeatedly verified in
practice with costly consolidated (rapidly solidified powder) RSP
alloys. Extensive deformation of injection molded material and the
like, to change grain boundary character, requires the special
kneading process that is accomplished in the present approach with
SWP. The other approaches available for such extensive deformation
(e.g. ECAP, high pressure torsion), do not appear suitable for
commercial scale-up, nor can they be easily automated for producing
thin, wide sheets.
[0123] Accordingly, via the present invention, an end resultant can
be produced, by initially providing a net-shape sheet bar alloy
with a uniform microstructure and an original fine grain size of
less than 10 .mu.m through rapid cooling during forming, without
segregation through the thickness of the material. This can be
achieved by various forming methods including injection molding and
other variations on injection molding, including semi-solid metal
injection molding, extrusion molding, TRC (hot rolled). Afterward,
the microstructure is refined to a nano-structure by processing the
sheet into an untextured sheet that exhibits superior formability.
This can be achieved by hot-pressing, rolling or other processes
utilizing appropriately shaped surfaces in the dies as previously
discussed. The final net-shaped part is thereby after formed by
either superplastic forming (SPF), warm drawing, warm stamping or
other methods. (Initial grain size may be reduced to lower the SPF
working stress, to lower the SPF temperature for better surface
finishes, and to raise the SPF rate.) One the net-shaped part is
formed, optional heat treating (annealing, etc.) may be done to the
final part to grow the grains so as to stop SPF and to impart creep
resistance to the final article. As a result, what is attained is
an inexpensive, light-weight part with very high strength to weight
ratio, along with enhanced toughness.
[0124] As seen above, the process starts with un-textured sheet
alloy having a fine grain size of less than 10 .mu.m. However, the
sheet alloy may be two phase and/or include high-angle grain
boundaries; the former to control grain growth, promote grain
boundary shear during SPF and strengthen the final part, and the
latter to promote final net shaping and decrease texture. In
refining the microstructure to obtain a grain size of about 1
micron, severe slip deformation is imparted to generate
simultaneous recrystallization to micron-sized grains faced with
high-angle grain boundaries. Thereafter, the coarse second phases
are further sub-divided and/or reprecipitated into nano-sized
arrays. In the above, twinning and the generation of textures are
both minimized.
[0125] As an example, a commercial AZ31B Mg alloy in the form of
hot-rolled plate, with thickness of 6.35 mm, was used as a
precursor material work piece. The chemical composition of this
alloy is 3.0 wt % Al, 1.0 wt % Zn, 0.45 wt % Mn and the balance Mg.
An 89.times.89 mm square work piece was cut from the as-received
plate, and then processed by SWP as described above. The initial
bimodal structure of the as-received alloy was refined into a
nearly uniform ultrafine grain structure. The bimodality of the
initial structure and its change toward a more uniform structure
were characterized by a detailed grain size distribution analysis
using known computer image analysis software. Based on image
analysis, the initial bimodal microstructure of the as-received
alloy contains 31% area fraction of coarse grains of size 22.1
.mu.m, but has an average grain size of 9.8 .mu.m. The final
microstructure after SWP had an average grain size of 1.4 .mu.m,
which contained less than 3% area fraction of coarse grains.
[0126] Mechanical properties of AZ31B Mg alloy for different alloy
processing conditions at room temperature are presented in Table 2
in terms of strength, elongations (including uniform and
post-uniform elongations), and normal anisotropy ratio (R). In
comparing tensile stress vs. strain for coarse grain as-received
and ultrafine grain as-processed alloy, it is seen that both
strength and ductility are greatly improved due to grain
refinement. The yield strength increases from 160 MPa to 280 MPa,
and the tensile elongation increases from 13.0% to 22.6%. The
presence of very coarse grain bands in the surface of the
as-received alloy appears not to change the yield strength, but
deteriorates the alloy's ductility. The post uniform strain depends
on strain rate sensitivity, m. The variation of m for the
as-received, as processed and annealed materials in terms of grain
size, and the corresponding uniform strains, show that the m value
decreases with increasing the grain size. The m value of the
as-processed alloy is found to be more than four times that of the
as-received alloy. This is a remarkable improvement at room
temperature. Uniform strain does not change with the grain size
monotonically. The ultrafine grained sample has the lowest uniform
strain. With grain size increasing, the uniform strain first rises,
and then decreases after a critical size of about 5 .mu.m.
[0127] Anisotropy in strength and ductility was also examined. For
in-plane compression, the as-received alloy yielded at a stress of
60 MPa, which is rather low. After a low rate of strain hardening
up to a strain of 0.05, strain hardening rate increased remarkably
with increasing strain (at this point the hardening curve is
concave). For in-plane tension and normal-to-plane compression, a
much higher yield stress and a lower rate of strain hardening were
observed. In addition, for normal-to-plane compression, the rapid
strain hardening and high yield strength caused failure of tested
specimen at a low strain (.epsilon.=0.06). For in-plane compression
of the as-processed alloy, the yield strength was lower than that
for tension, but a significant increase in strength occurred in
comparison with the as-received alloy, and no concave curve for
strain hardening behavior is observed. The difference in the yield
strength for in-plane tension and compression decreased as grain
size becomes finer. Similar behavior is seen in terms of strain
hardening. This suggests that the anisotropy in strength for
in-plane deformation decreases due to grain refinement. In
normal-to-plane compression of both, the as-processed and
as-received materials have similar yield strength, but the ultimate
tensile strength for the fine grain processed material is higher
indicating that strain localization in the coarse grain alloy
causes a premature peak in the flow stress. TABLE-US-00002 TABLE 2
AZ31B Normal Mg Alloy Tensile yield Ultimate tensile Anisotropy
(room temperature) strength, MPa strength, MPa Elongation*, %
e.sub.u**, % e.sub.pu**, % Ratio (R) As-Received # 160 274 13.0
(13.5) 11.9 1.1 3.8 As-Processed 280 308 22.6 (29.0) 8.4 14.2 6.0
As-Processed + 218 271 24.4 (32.3) 13.4 11.1 5.0 Annealed at
250.degree. C. *Reported elongation is over 12.7 mm gauge length. A
shorter gauge length of 5.0 mm gives a higher value of elongation
shown in the parenthesis. **e.sub.u and e.sub.pu refer to uniform
strain and post-uniform strain, respectively. # For the as-received
material, mechanical test data are from interior region of plate
(fine grain region)
[0128] Table 2 shows that the fine grain as-processed alloy has
improved mechanical properties such as higher tensile yield
strength and higher post-uniform elongation, and higher (R) value.
Annealing increases tensile elongation values further. When
examined for microstructural changes, no twinning was observed in
the processed material. Further, the as-received alloy displayed a
rough surface similar to "orange peel" white effect, the fine grain
processed alloy exhibited a smooth surface after the test. In
addition, the degree of necking is found more gradual in the
as-processed alloy.
[0129] As noted above, grain refinement increases yield strength
and reduces strain hardening rate in the fine grain processed alloy
in comparison to coarse grain as-received alloy. The high strain
rate sensitivity, m, found in the fine grain condition has no
connection with texture, but rather is related to the many grain
boundaries present in the structure, and it is well-known that a
higher m promotes increased elongation by delaying the tendency for
strain localization. In the fine grain alloy no twinning was seen,
therefore it is believed that dislocation process primarily
accommodates plastic deformation in the fine grain alloy. Yield
strength for the fine grain alloy is higher due to its finer
structure (i.e., smaller spacing between barriers to dislocation)
and large fraction of grain (or subgrain) boundaries. More grain
boundaries assist in the dynamic recovery process. This, combined
with a tendency for shearing or sliding along grain boundaries,
increases strain rate sensitivity in the fine grain alloy.
[0130] It appears that there is a unique relationship between m and
post-uniform elongation, showing that a high value of sensitivity m
significantly enhances post-uniform elongation. This relationship
appears independent of the alloy system, twinning effect, and
texture effect. AZ31B magnesium generally fits this trend, however
annealed data shows somewhat higher post-uniform elongation,
possibly because of its higher strain hardening capacity.
[0131] The somewhat stronger basal texture in the processed alloy
may not fully explain the large difference in R-value between these
two different grain size conditions. Annealing the as-processed
alloy, which appears to not change texture, increases grain size
and decreases R-value. It is found that fine grain processing has
contributed to changes in R-value, but a reduced value of R did not
decrease tensile elongation. Thus annealing effect primarily
reduces internal stresses. In the fine grain processed material,
the presence of a larger fraction of grain boundary area may favor
the activation of non-basal slip for compatibility reasons. Cross
slip of prismatic or pyramidal <a> dislocations can promote
high R-value in basal textured sheet for in-lane tensile
deformation, while extensive <c+a> slip may not increase
because a decrease in the R-value, particularly if shearing along
many grain boundaries occurs. Thus, non-basal <a> slip may be
favored in the fine grain processed alloy for in-plane tension.
[0132] In the fine grained alloy, the yield strength for in-plan
tension is found to be greater than that for compression even
though twinning is inhibited. For metal with limited number of slip
systems such as Mg, grain boundary regions experience a greater
degree of strain incompatibility and more complex loading than in
cubic metals. In ultra-fine grain Mg, where many grain boundaries
lead to many locations for incompatibility, changes in local
stress-state and stress-concentration can be a significant
contributor to the deformation mechanics of polycrystal.
[0133] As a further example, an Mg-9Al alloy (AZ91D) sheet bar
measuring 100.times.150.times.3 mm was semi-solid metal injection
molded in a commercial 280 ton Thixomolding.RTM. machine at
Thixomat, Inc. (Ann Arbor, Mich.). This sheet bar was pressed at
190.degree. C. between opposing sine-wave dies having a corrugated
surface pattern through 4 cycles, turning the sheet 900 between
cycles. The sheet was press flattened after the 4.sup.th pressing
cycle. The total reduction of thickness was from 3 mm to 0.8 mm,
i.e. 73%. The resultant tensile strengths are compared to
commercial AZ31 (Mg-3Al) sheet in Table 3. TABLE-US-00003 TABLE 3
Material 0.2% YS, MPa UTS, MPA AZ91D, as injection molded 150 220
sheet bar AZ91D, SWP 4 Cycles 260 300 AZ31, commercial sheet* 150
255 *ASM Handbook
[0134] As seen from Table 3, yield strength was increased by 73%
compared to the original sheet bar and the commercial AZ31.
Ultimate tensile strength was respectively increased 36% and
18%.
[0135] This resultant SWP sheet was then annealed at 150 or
250.degree. C. Hardness of the fine grained material derived from
the original liquid phase in the as-semi-solid metal injection
molded, SWP, SWP+rolled/annealed state were measured and the
results are presented in Table 4. TABLE-US-00004 TABLE 4 Material
Microhardness, MPa AZ91D, as SSMI molded, 5 .mu.m grain size 772
AZ91D, SWP 932 AZ91D, SWP + Annealed @ 150.degree. C. 958 AZ91D,
SWP + Annealed @ 250.degree. C. 858 AZ31, commercial sheet, 10
.mu.m grain size 600 AZ31, commercial sheet, 1 .mu.m grain size
720
[0136] The fine grained original liquid region of the as semi-solid
metal injection molded sheet bar had a 772 MPa hardness, which was
increased to 932 MPa by SWP. Annealing at 150.degree. C. increased
the hardness further to 958 MPa. Compared to previous data from
AZ31, as presented in the graph of FIG. 7, the SWP material from
AZ91 D was harder than equivalent grain size AZ31. Part of this
hardness increment over AZ31 is believed attributable to nano-size
.beta. phase in the Al rich AZ91 D alloy. Microstructures confirmed
that the coarse .beta. phase of the starting material had been
sub-divided and reprecipitated as nano-particles, some at grain
boundaries.
[0137] The feasibility of SPF of the SWP sheet has also been
demonstrated by the inventors. As FIG. 8 demonstrates, the depth of
cup produced via a bulge test of the SWP AZ91D is deeper than that
of a sheet (of corresponding thickness) of the starting material
formed by Thixomolding (semi-solid metal injection molding process
of Thixomat, Inc., Ann Arbor, Mich.) only. In fact, the depth is
much greater than that formed in commercial 10-20 .mu.m AZ31
sheet.
[0138] Although corrosion was not specifically tested, the 9% Al
level of SWP AZ91D should be quite superior to commercial AZ31 (3%
Al) in resisting exposure to road or other aqueous
environments.
[0139] While illustrated above with Mg alloy, other alloys, capable
of being processed into a precursor work piece having an initial
fine grain structure, are believed to be suitable to the present
invention and include, without limitation, Al, Zn, Ni, Cu,
.alpha./.beta. Ti, steels, duplex .alpha./.gamma. stainless steels,
.alpha./.gamma. steels, .gamma./martensite Maraging steels and
metal/ceramic particle composites.
[0140] Potential markets for products manufactured by the present
invention are envisioned in the automotive and aerospace fields,
among others, where weight savings can be gained by replacing steel
and aluminum with magnesium. Complex 3-D net-shapes can be SPF to
greatly reduce the number of sub-assemblies and the costs of
multiple fabrication and assembly. High tensile strength and high
toughness will be attained by sub-micron grain sizes, second phase
nanocrystals and by the selection of ductile alloys. The unique
microstructure so attained will greatly reduce texture and its
usual barrier to formability.
[0141] Automobile companies are predicting very significant
increases in Mg tonnage for automotive vehicles, as much growth as
from 5 Kg/car up to 200 Kg/car. There is a need to enable the
United States automotive industry to lead this sea change in
light-weighting. Additional markets should further open in the
aerospace, defense and other industries.
[0142] SWP of suitable alloys should reduce the cost of making thin
sheet material by eliminating multiple stages of rolling and
annealing. Deformation by SWP changes the grain boundary character
and increases the ability to be formed by warm forming or by
superplastic deformation. If sinusoidal deformation is carried out
immediately following injection molding, the sensible heat in the
molded blank can be utilized. Following immediate rolling or
pressing of the sheet bar, it can be formed by SPF into complex
part shapes. Such forming can be accomplished at 200.degree. C.
Thus, the entire component fabrication technology can be set into a
continuous operation without storage of coils of sheets,
considerable coil annealing, coiling and uncoiling operations. The
removal of all of the steps involved with coiling and cranes
handling transport of coils would minimize investment in plants. A
leaner manufacturing process for parts would emerge.
[0143] It is envisioned that SWP can be accomplished by integrating
injection molding machines for metal with conventional pressing and
rolling equipment and should be feasible on process equipment
already used in the aerospace and automotive industries. Deep
drawing also can be practiced on conventional presses.
[0144] As a person skilled in the art will readily appreciate, the
above description is meant as an illustration of implementations of
the principles of this invention. This description is not intended
to limit the scope or application of this invention in that the
invention is susceptible to modification, variation and change,
without departing from spirit of this invention, as defined in the
following claims.
* * * * *