U.S. patent application number 11/679239 was filed with the patent office on 2008-01-03 for apparatus and method of producing a fine grained metal sheet for forming net-shape components.
Invention is credited to Raymond F. Decker, Amit Ghosh, Sanjay Kulkami, Bilal Mansoor.
Application Number | 20080000557 11/679239 |
Document ID | / |
Family ID | 39739888 |
Filed Date | 2008-01-03 |
United States Patent
Application |
20080000557 |
Kind Code |
A1 |
Ghosh; Amit ; et
al. |
January 3, 2008 |
APPARATUS AND METHOD OF PRODUCING A FINE GRAINED METAL SHEET FOR
FORMING NET-SHAPE COMPONENTS
Abstract
A method and apparatus for producing ultra-fine grained
magnesium metal alloy material sheets. The apparatus molds and
rapidly solidifies a metal alloy material to form a fine grain
precursor. The precursor is then subjected to deformation 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) ;
Kulkami; Sanjay; (Livonia, MI) ; Mansoor; Bilal;
(Ann Arbor, MI) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Family ID: |
39739888 |
Appl. No.: |
11/679239 |
Filed: |
February 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11425067 |
Jun 19, 2006 |
|
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11679239 |
Feb 27, 2007 |
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Current U.S.
Class: |
148/420 ;
148/522; 148/557; 164/335 |
Current CPC
Class: |
B22F 2998/00 20130101;
B22F 2998/00 20130101; B22D 21/007 20130101; B22F 2998/00 20130101;
C22F 1/06 20130101; C22F 3/00 20130101; C22C 49/04 20130101; C22C
23/02 20130101; C22C 47/20 20130101; B22D 17/007 20130101 |
Class at
Publication: |
148/420 ;
148/522; 148/557; 164/335 |
International
Class: |
C22C 23/00 20060101
C22C023/00; B22D 45/00 20060101 B22D045/00; C22F 1/06 20060101
C22F001/06; C22F 3/00 20060101 C22F003/00 |
Claims
1. A method of forming a sheet material having a refined grain
structure, the method comprising: providing a magnesium metal alloy
material; molding and rapidly solidifying the metal alloy material
to form a fine grain precursor, wherein molding includes
substantially melting the metal alloy material; and imparting
plastic deformation to the fine grain precursor by a deformation
strain to form an ultra fine grain structured sheet form.
2. The method of claim 1 wherein the fine grain precursor has an
isotropic grain structure.
3. The method of claim 1 wherein the step of providing the
magnesium metal alloy material and the step of molding and rapidly
solidifying are repeated to form a plurality of fine grain
precursors and the method further comprises stacking the plurality
of fine grain precursors to form a stack and the step of imparting
plastic deformation includes plastically deforming the stack by the
deformation strain.
4. The method of claim 3 wherein a ratio of a thickness of the
stack to a thickness of the ultra fine grain structured sheet form
is in the range of approximately 3:1 to 30:1.
5. The method of claim 3 wherein a ratio of a plan view area of the
ultra fine grain structured sheet form to a plan view area of the
stack is in the range of approximately 3:1 to 30:1.
6. The method of claim 3 wherein the step of imparting plastic
deformation bonds the fine grain precursors together to form the
ultra fine grain structured sheet form.
7. The method of claim 3 wherein at least two of the fine grain
precursors are molded from respectively different metal alloys
having correspondingly different properties.
8. The method of claim 7 wherein at least one of the fine grain
precursors has comparatively more corrosion resistant than another
fine grain precursor.
9. The method of claim 7 wherein at least one of the fine grain
precursors has comparatively higher elongation than another fine
grain precursor.
10. The method of claim 7 wherein at least one of the fine grain
precursors has comparatively higher strength than another fine
grain precursor.
11. The method of claim 6 wherein reinforcing elements are disposed
between the fine grain precursors to form a composite ultra fine
grain structured sheet form.
12. The method of claim 11 wherein the reinforcing elements are
selected from the group consisting of whiskers, graphite fibers,
ceramic fibers, wires, wire mesh and metal fibers.
13. The method of claim 1 wherein rapidly solidifying the metal
alloy material is at a cooling rate of at least 80 C/sec to form
the fine grain precursor.
14. The method of claim 1 wherein the fine grain precursor has a
thickness not exceeding about 4 mm.
15. The method of claim 1 wherein the fine grain precursor has a
total porosity not exceeding about 2 percent.
16. The method of claim 1 wherein the fine grain precursor has a
gas porosity not exceeding about 1 percent.
17. The method of claim 1 wherein the deformation strain is at a
strain rate and the step of imparting plastic deformation is
performed while the fine grain precursor is heated to a
temperature, wherein the strain rate, the temperature and the
deformation strain cooperate to recrystallize the fine grain
precursor to the ultra fine grain structured sheet form.
18. The method of claim 17 wherein the fine grain precursor is
recrystallized by a mechanism including continuous dynamic
recrystallization producing the ultra fine grain structured sheet
form with at least 50 percent high angle boundaries.
19. The method of claim 17 wherein the ultra fine grain structured
sheet form has an intensity of basal [0002] texture not exceeding
about 5.
20. The method of claim 17 wherein the ultra fine grain structured
sheet form has a yield strength anisotropy not exceeding about 10
percent.
21. The method of claim 17 wherein the strain rate is in the range
of approximately 0.1 to 50 s.sup.-1.
22. The method of claim 17 wherein the temperature is in the range
of approximately 150 C to 450 C.
23. The method of claim 17 wherein the strain rate ({acute over
(.epsilon.)}) and the temperature (T) produce a Zener factor (Z) of
greater than about 10.sup.9 s.sup.-1 as determined by the formula
Z={{acute over (.epsilon.)}.times.exp(Q/RT)}.sup.-0.2, where Q is
the activation energy (135 kj mol.sup.-1), and R is the gas
constant.
24. The method of claim 17 wherein the deformation strain is at
least 0.5.
25. The method of claim 17 wherein imparting plastic deformation
occurs substantially by slip between grain boundaries of the fine
grain precursor with less than about 10 percent twinning of the
grain structure.
26. The method of claim 17 wherein imparting plastic deformation
occurs without substantial shear banding of the grain
structure.
27. The method of claim 1 wherein the step of molding and
solidifying develops a multiphased microstructure in the fine grain
precursor.
28. The method of claim 27 wherein the multiphased microstructure
includes pinning particles that minimize grain growth.
29. 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.
30. The method of claim 1 wherein the molding step and the
imparting plastic deformation step are performed in an integrated
apparatus.
31. The method of claim 1 wherein the molding step and the
imparting plastic deformation step are performed by separate
machines.
32. The method of claim 1 wherein the molding step includes
semisolid metal injection molding of the metal alloy material.
33. The method of claim 32 wherein a solids content of the
semisolid metal material does not exceed about 30 percent.
34. The method of claim 32 wherein a solids content of the
semisolid metal material does not exceed about 10 percent.
35. The method of claim 32 wherein the semisolid metal injection
molding includes delivering the semisolid metal material to a mold
via a hot runner system.
36. The method of claim 35 wherein a plurality of the fine grain
precursors are formed with at least 80 percent production
yield.
37. The method of claim 32 wherein the semisolid metal material is
injected with a screw shot velocity of at least 1.5 m/sec.
38. The method of claim 32 wherein the molding step further
includes providing argon gas to the metal alloy material.
39. The method of claim 1 wherein the molding step further includes
extruding of the metal alloy material.
40. The method of claim 1 wherein the molding step further includes
vacuum molding of the metal alloy material.
41. The method of claim 1 further comprising, after the step of
imparting plastic deformation, the step of net shaping the ultra
fine grain structured sheet to form a part.
42. The method of claim 41 further comprising the step of heat
treating the net shaped part to impart creep resistance to the net
shaped part.
43. The method of claim 41 wherein the step of net shaping includes
one of stamping, drawing, deep drawing and superplastic
forming.
44. The method of claim 41 wherein the step of net shaping forms an
automotive component.
45. An apparatus for performing the method of claim 1.
46. An article formed by the method of claim 1.
47. The method of claim 1 wherein the step of imparting plastic
deformation includes die pressing of the fine grain precursor.
48. The method of claim 1 wherein the step of imparting plastic
deformation includes rolling the fine grain precursor.
49. The method of claim 48 wherein the step of imparting plastic
deformation further includes constraining edges of the fine grain
precursor.
50. The method of claim 49 wherein the edges of the fine grain
precursor are constrained by a Turks Head arrangement.
51. The method of claim 1 wherein the step of imparting plastic
deformation includes rolling the fine grain precursor in a
plurality of rolling passes with a plurality of respective
deformation strains.
52. The method of claim 51 wherein the corresponding deformation
strain of each rolling pass is at least 50 percent.
53. The method of claim 52 wherein the step of rolling includes a
first rolling pass at a temperature above ambient, wherein each
successive pass is at a lower temperature.
54. The method of claim 52 wherein the plurality of rolling passes
are cross rolled.
55. The method of claim 1 wherein the step of imparting plastic
deformation includes extrusion of the fine grain precursor.
56. The method of claim 1 wherein the step of imparting plastic
deformation includes forging of the fine grain precursor.
57. The method of claim 1 wherein the step of imparting plastic
deformation includes flow forming of the fine grain precursor.
58. The method of claim 1 wherein the sheet form is provided having
a grain structure of less than about 5 micrometers.
59. The method of claim 1 wherein the sheet form is provided having
a grain structure of less than about 2 micrometers.
60. The method of claim 1 wherein the sheet form is provided having
a grain structure of less than about 1 micrometer.
61. The method of claim 1 wherein the precursor is provided having
a grain structure of less than about 10 micrometer.
62. The method of claim 1 wherein the precursor is provided having
a grain structure of less than about 5 micrometer.
63. The method of claim 1 wherein the step of imparting plastic
deformation is performed while the precursor is heated above
ambient.
64. The method of claim 1 wherein the magnesium metal alloy is
provided with a moisture content less than about 0.1 percent.
65. The method of claim 1 wherein the step of imparting plastic
deformation includes: plastically deforming the fine grain
precursor by a combination of alternating tensile strain and
compressive strain to form an SWP sheet, wherein the steps of
providing a metal material, molding and rapidly solidifying and
plastically deforming are repeated to form a plurality of SWP
sheets; stacking the plurality of SWP sheets to form a SWP stack;
and plastically compressing the SWP stack to form the ultra fine
grain structured sheet form.
66. The method of claim 65 wherein the step of plastically
deforming includes corrugating the fine grain precursor in a first
direction and subsequently corrugating the fine grain precursor in
a second direction.
67. The method of claim 66 wherein the step of plastically
deforming the fine grain precursor further includes flattening the
corrugated fine grain precursor.
68. The method of claim 67 wherein the compressive strain is
imparted at least in part by flattening the work piece while
constraining lengthening of the work piece in at least one
direction.
69. 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 a 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; and plastic
deformation means including a pair of opposing forming members for
imparting deformation strain into the precursor article forming a
sheet of the metal material having an ultra-fine grain size.
70. The apparatus of claim 69 wherein the opposed forming members
are dies.
71. The apparatus of claim 69 wherein the opposed forming members
are rolls.
72. The apparatus of claim 69 further including means for stacking
a plurality of the precursor articles into a stack, and wherein the
pair of opposing forming members are configured for imparting
deformation strain into the stack to form the sheet of the metal
material having the ultra fine grain size.
73. The apparatus of claim 72 wherein the plastic deformation means
further including 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 second pair of opposing forming members having
protrusions formed on a surface thereof, the protrusions of one
second forming member being offset from the protrusions of the
opposing second forming member; the plastic deformation means
further including flattening means for flattening the corrugated
work piece, wherein the stacking means stacks a plurality of the
flattened work pieces to form the stack.
74. The apparatus of claim 73 wherein the second opposed forming
members are dies.
75. The apparatus of claim 73 wherein the second opposed forming
members are rolls.
76. The apparatus of claim 72 wherein the stacking means further
including means for disposing reinforcing elements between the
precursor articles.
77. The apparatus of claim 72 wherein the stacking means further
includes means for arranging the precursor articles in a
pre-determined position.
78. The apparatus of claim 69 further comprising net shaping means
for shaping the sheet form of the metal material into a net-shaped
article.
79. The apparatus of claim 78 wherein the shaping means is one of a
drawing press and a superplastic forming machine.
80. The apparatus of claim 69 wherein the receptacle, feeder,
heating means, discharge means and forming means are part of an
injection molding machine.
81. The apparatus of claim 69 wherein the receptacle, feeder,
heating means, discharge means and forming means are part of a
semi-solid metal injection molding machine.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] The present invention relates to producing net shaped
components of increased strength. More particularly, the invention
relates to producing magnesium alloy sheet components, having
micrometer sized grain structures, that can be subsequently used in
the production of net shaped sheet components of increased
strength.
[0003] 2. Related Technology
[0004] 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.
[0005] 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 AZ91
D, still dominate the tonnage of commercial sheet and casting
markets.
[0006] Hexagonal close packed (HCP) structured Mg alloys have low
symmetry of slip systems that contributes to high anisotropy of
mechanical properties. Slip is a crystallographic shear process
associated with dislocation glide that underlies the large plastic
deformation of crystalline metals. 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. Twinning is a deformation mechanism in which
small, often plate or lens shaped, regions of a crystal or grain
reorient crystallographically to adopt a twin relationship to the
parent crystal. 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.
[0007] 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 cannot 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 the application of wrought Mg
alloys. First the symmetry of hexagonal close packed crystal
structures has the effect of limiting the number of independent
slip systems, thus providing alloys with poor formability and
ductility near room temperature. Second, forming 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
recrystallization 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 a
deformation strain, such as for example, tension-compression or
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 (0.3m 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, such as for example, "earring", where
shapes resembling ears are formed in portions of the grain
microstructure. 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 OF THE INVENTION
[0018] In achieving the above object, the inventors have discovered
a practical new process and apparatus to generate inexpensive ultra
fine grain structured sheet comprising a magnesium metal alloy,
where grain sizes of less than or equal to about 2 .mu.m are
achieved, which can be subsequently deformed via superplastic
forming processes, or any other suitable forming process, to form
net shaped, sheet formed articles.
[0019] The present process involves the deformation strain
processing 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 and extrusion molding. 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 grain precursor having
a grain size of less than about 10 .mu.m. Thereafter, the fine
grain precursor is subjected to deformation straining, which may
include for example, tension-compression, compression and/or
sine-wave deformation (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 grain structure, the method comprising
the steps of: providing a magnesium metal alloy material; molding
and rapidly solidifying the metal alloy to form a fine grain
precursor, wherein molding includes substantially melting the metal
alloy material, and imparting plastic deformation to the fine grain
precursor by a deformation strain to form an ultra fine grain
structured sheet form.
[0020] In another aspect, the fine grain precursor has an isotropic
grain structure.
[0021] In another aspect, the step of providing the magnesium metal
alloy material and the step of molding and rapidly solidifying are
repeated to form a plurality of fine grain precursors and the
method further comprises stacking the plurality of fine grain
precursors to form a stack and the step of imparting plastic
deformation includes plastically deforming the stack by the
deformation strain.
[0022] In another aspect, a ratio of a thickness of the stack to a
thickness of the ultra fine grain structured sheet form is in the
range of approximately 3:1 to 30:1.
[0023] In yet another aspect, a ratio of a plan view area of the
ultra fine grain structured sheet form to a plan view area of the
stack is in the range of approximately 3:1 to 30:1.
[0024] It is of further aspect, the step of imparting plastic
deformation bonds the fine grain precursors together to form the
ultra fine grain structured sheet form.
[0025] Still another aspect is that at least two of the fine grain
precursors are molded from respectively different metal alloys
having correspondingly different properties.
[0026] A further aspect is that at least one of the fine grain
precursors has comparatively more corrosion resistant than another
fine grain precursor.
[0027] In another aspect, at least one of the fine grain precursors
has comparatively higher elongation than another fine grain
precursor.
[0028] In a further aspect, at least one of the fine grain
precursors has comparatively higher strength than another fine
grain precursor.
[0029] In yet another aspect, reinforcing elements are disposed
between the fine grain precursors to form a composite ultra fine
grain structured sheet form.
[0030] It is also another aspect that the reinforcing elements are
selected from the group consisting of whiskers, graphite fibers,
ceramic fibers, wires, wire mesh and metal fibers.
[0031] In yet another aspect, rapidly solidifying the metal alloy
material is at a cooling rate of at least 80 C/sec to form the fine
grain precursor.
[0032] It is also another aspect the fine grain precursor has a
thickness not exceeding about 4 mm.
[0033] Still another aspect is that the fine grain precursor has a
total porosity not exceeding about 2 percent.
[0034] A further aspect is that the fine grain precursor has a gas
porosity not exceeding about 1 percent.
[0035] In another aspect the deformation strain is at a strain rate
and the step of imparting plastic deformation is performed while
the fine grain precursor is heated to a temperature, wherein the
strain rate, the temperature and the deformation strain cooperate
to recrystallize the fine grain precursor to the ultra fine grain
structured sheet form.
[0036] In a further aspect, the grain structure is recrystallized
by a mechanism including continuous dynamic recrystallization
producing the ultra fine grain structure with at least 50 percent
high angle boundaries.
[0037] In yet another aspect, the ultra fine grain structured sheet
form has an intensity of basal (0002) texture not exceeding about
5.
[0038] In another additional aspect, the ultra fine grain
structured sheet form has a yield strength anisotropy not exceeding
about 10 percent.
[0039] In another aspect, the deformation strain rate is in the
range of approximately 0.1 to 50 s.sup.-1.
[0040] In still another aspect, the temperature is in the range of
approximately 150 C to 450 C.
[0041] In another aspect, the strain rate ({acute over
(.epsilon.)}) and the temperature (T) produce a Zener factor (Z) of
greater than about 10.sup.9 s.sup.-1 as determined by the formula
Z={{acute over (.epsilon.)}.times.exp(Q/RT)}.sup.-0.2, where Q is
the activation energy (135 kj mol.sup.-1), and R is the gas
constant.
[0042] In another aspect, the deformation strain is at least
0.5.
[0043] In yet another aspect, imparting plastic deformation occurs
substantially by slip between grain boundaries of the fine grain
precursor with less than about 10 percent twinning of the grain
structure.
[0044] In another aspect, imparting plastic deformation occurs
without substantial shear banding of the grain structure.
[0045] In another aspect, the step of molding and solidifying
develops a multiphased microstructure in the fine grain
precursor.
[0046] In an additional aspect, the multiphased microstructure
includes pinning particles that minimize grain growth.
[0047] In another aspect, 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.
[0048] In another aspect, the molding step and the imparting
plastic deformation step are performed in an integrated
apparatus.
[0049] In still another aspect, the molding step and the imparting
plastic deformation step are performed by separate machines.
[0050] In an additional aspect, the molding step includes semisolid
metal injection molding of the metal material.
[0051] In one aspect, a solids content of the semisolid metal
material does not exceed about 30 percent.
[0052] In another aspect, a solids content of the semisolid metal
material does not exceed about 10 percent.
[0053] In yet another aspect, the semisolid metal injection molding
includes delivering the semisolid metal material to a mold via a
hot runner system.
[0054] In still another aspect, a plurality of the fine grain
precursors are formed with at least 80 percent production
yield.
[0055] In another aspect, the semisolid metal material is injected
with a screw shot velocity of at least 1.5 m/sec.
[0056] In another aspect, the molding step further includes
providing argon gas to the metal alloy material.
[0057] In a further aspect, the molding step further includes
extruding of the metal alloy material.
[0058] In yet another aspect, the molding step further includes
vacuum molding of the metal alloy material.
[0059] It is also an aspect the method further comprises, after the
step of imparting plastic deformation, the step of net shaping the
nano-sized grain structure sheet.
[0060] Still another aspect the method further comprises the step
of heat treating the net shaped part to impart creep resistance to
the net shaped part.
[0061] A further aspect the step of net shaping includes one of
stamping, drawing, deep drawing and superplastic forming.
[0062] In another aspect, the step of net shaping forms an
automotive component.
[0063] In a further aspect, an apparatus for performing the method
is provided.
[0064] In yet another aspect, an article formed by the method is
provided.
[0065] It is also an aspect that the step of imparting plastic
deformation includes die pressing of the fine grain precursor.
[0066] Still another aspect that the step of imparting plastic
deformation includes rolling the fine grain precursor.
[0067] Still another aspect that the step of imparting plastic
deformation further includes constraining edges of the fine grain
precursor.
[0068] Still another aspect that the edges of the fine grain
precursor are constrained by a Turks Head arrangement.
[0069] A further aspect the step of imparting plastic deformation
includes rolling the fine grain precursor in a plurality of rolling
passes with a plurality of respective deformation strains.
[0070] In another aspect, the corresponding deformation strain of
each rolling pass is at least 50 percent.
[0071] In a further aspect, the step of rolling includes a first
rolling pass at a temperature above ambient, wherein each
successive pass is at a lower temperature.
[0072] In yet another aspect, the plurality of rolling passes are
cross rolled.
[0073] In an additional aspect, the step of imparting plastic
deformation includes extrusion of the fine grain precursor.
[0074] In another aspect, the step of imparting plastic deformation
includes forging of the fine grain precursor.
[0075] In still another aspect, the step of imparting plastic
deformation includes flow forming of the fine grain precursor.
[0076] In another aspect, the sheet form is provided having a grain
structure of less than about 5 micrometers.
[0077] In another aspect, the sheet form is provided having a grain
structure of less than about 2 micrometers.
[0078] In yet another aspect, the sheet form is provided having a
grain structure of less than about 1 micrometer.
[0079] In another aspect, the precursor is provided having a grain
structure of less than about 10 micrometer.
[0080] In another aspect, the precursor is provided having a grain
structure of less than about 5 micrometer.
[0081] In an additional aspect, the step of imparting plastic
deformation is performed while the precursor is heated above
ambient.
[0082] In still another aspect, the magnesium metal alloy is
provided with a moisture content less than about 0.1 percent.
[0083] In another aspect, the step of imparting plastic deformation
includes plastically deforming the fine grain precursor by a
combination of alternating tensile strain and compressive strain to
form an SWP sheet, wherein the steps of providing a metal material,
molding and rapidly solidifying and plastically deforming are
repeated to form a plurality of SWP sheets; stacking the plurality
of SWP sheets to form a SWP stack; and plastically compressing the
SWP stack to form the ultra fine grain structured sheet form.
[0084] In a further aspect, the step of plastically deforming
includes corrugating the fine grain precursor in a first direction
and subsequently corrugating the fine grain precursor in a second
direction.
[0085] In another aspect, the step of plastically deforming the
fine grain precursor further includes flattening the corrugated
fine grain precursor.
[0086] In still another aspect, the compressive strain is imparted
at least in part by flattening the work piece while constraining
lengthening of the work piece in at least one direction.
[0087] In another aspect, an apparatus for refining grain structure
and producing ultra-fine grained metal material sheets, wherein the
apparatus comprises a receptacle having an inlet, a discharge
cutlet 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 a 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, and plastic deformation means including a pair
of opposing forming members for imparting deformation strain into
the precursor article forming a sheet of the metal material having
an ultra-fine grain size.
[0088] In another aspect, the opposed forming members are dies.
[0089] In further another aspect, the opposed forming members are
rolls.
[0090] In another aspect, the apparatus further includes means for
stacking a plurality of the precursor articles into a stack, and
wherein the pair of opposing forming members are configured for
imparting deformation strain into the stack to form the sheet of
the metal material having the ultra fine grain size.
[0091] In another aspect, the plastic deformation means further
including 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
second pair of opposing forming members having protrusions formed
on a surface thereof, the protrusions of one second forming member
being offset from the protrusions of the opposing second forming
member; the plastic deformation means further including flattening
means for flattening the corrugated work piece, wherein the
stacking means stacks a plurality of the flattened work pieces to
form the stack.
[0092] In still another aspect, the second opposed forming members
are dies.
[0093] In another aspect, the second opposed forming members are
rolls.
[0094] In an additional aspect, the stacking means further
including means for disposing reinforcing elements between the
precursor articles.
[0095] In yet another aspect, the stacking means further includes
means for arranging the precursor articles in a pre-determined
position.
[0096] In another aspect, the apparatus further comprises net
shaping means for shaping the sheet form of the metal material into
a net-shaped article.
[0097] In another aspect, the shaping means is one of a drawing
press and a superplastic forming machine.
[0098] In a further aspect, the receptacle, feeder, heating means,
discharge means and forming means are part of an injection molding
machine.
[0099] In another aspect, the receptacle, feeder, heating means,
discharge means and forming means are part of a semi-solid metal
injection molding machine.
BRIEF DESCRIPTION OF THE DRAWINGS
[0100] FIG. 1 is a schematic illustration of a manufacturing cell
and method embodying the principles of the present invention;
[0101] FIG. 2 is a side view of roll dies in accordance with an
embodiment of the present invention;
[0102] FIG. 3 is a schematic illustration of a manufacturing cell
and method embodying the principles of the present invention;
[0103] FIG. 4 is a schematic illustration of a manufacturing cell
and method embodying the principles of the present invention;
[0104] FIG. 5A is a perspective view of the longitudinal roll dies
as seen in FIG. 4 and used in connection with the present
invention:
[0105] FIG. 5B is perspective view of transverse roll dies as seen
in FIG. 4 and used in connection with the present invention;
[0106] FIG. 6 is a flowchart of one possible process in accordance
with the present invention;
[0107] FIG. 7 is a diagrammatic illustration of the present
invention incorporating an extrusion device;
[0108] FIG. 8 is a graphical comparison of the effect of grain size
(d) on hardness (Hr) for SWP AZ91D and AZ31B; and
[0109] FIG. 9 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.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0110] According to one aspect and embodiment of the present
invention, a fine grain precursor is formed by the injection
molding (IM) of metal, such as by the Thixomolding.TM. process of
Thixomat, Inc., Ann Arbor, Mich. With use of 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, that is they have a homogeneous microstructure, with 4
to 5 .mu.m grain size a phase. Moreover, these injection molded Mg
alloys have non-columnar grains with less gas and shrink porosity.
Through the use of multiple feeding ports, the rapid injection
molding of large sheet bars is possible. Moreover, a hot runner
system may be employed for delivery of the liquid metal to a mold
for solidification, which may improve production yields of the
large sheet bars. Suitable sheet bar would be readily molded in
existing commercial Thixomolding machines, of sizes up to 1000
tons, with sheet dimensions up to about 5.times.400.times.400
mm.
[0111] Table 1 compares current production methods to the present
invention (IM+SWP and IM+Deformation Straining) for a precursor
work piece, such as sheet bar, as well as for a range of resulting
grain sizes. 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 .sup. .ltoreq.1
Injection Molded + As Deformation Strained .ltoreq.1-2 Deformation
Strain
[0112] 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
or other means. 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 substantially melted state such
that the feedstock material is injectable. 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. In at least one
embodiment the screw 18 shot velocity is at least 1.5
meters/second. A hot runner system (not shown) may optionally be
used to assist delivery of the material to the mold cavity thereby
minimizing any heat loss. Moreover, because this process may result
in a "frozen plug", that is the metal solidifies where the mold
receives the injectable material, pulling a vacuum on the mold
during molding is feasible and may further be used to decrease
resulting porosity of the 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.
[0113] 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; 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.
[0114] 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.
[0115] In another preferred embodiment, the liquid phase material
in the mold is rapidly solidified at a cooling rate of at least 80
C/second.
[0116] In another preferred embodiment, the sheet bar 30 has a
thickness not exceeding about 4 mm.
[0117] In another preferred embodiment, metallurgical process of
the machine 10 results in the sheet bar 30 having a total porosity
not exceeding about 2 percent. The total porosity comprises both a
shrinkage porosity, which is derived from shrinkage of the metal,
and a gas porosity. Shrinkage porosity comprises voids which are
more linear or flattened shaped and formed in the eutectic regions
around the grain boundaries, whereas gas porosity comprises voids
which are more spherically shaped and formed between the grain
boundaries.
[0118] In another preferred embodiment, the protective argon
atmosphere with a feedstock moisture content of less than about 0.1
percent, minimize gas porosity to not exceed 1 percent with minimal
formation of oxides.
[0119] Once the fine grained sheet bar 30 is formed, it is
subjected to a deformation strain. The deformation strain may be,
for example, a tensile-compressive strain, a compressive strain or
a strain or combination of strains otherwise defined by a strain
tensor or tensors. In at least one other embodiment, the
deformation strain involves imparting plastic deformation by at
least compressively straining the sheet bar 30. This second step of
deformation straining 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.
[0120] In one implementation of this deformation process, the sheet
bar 30 is heated to a temperature during deformation straining at a
strain rate. The temperature in combination with the deformation
strain and the strain rate cooperate to recrystallize the grain
structure to an ultra fine grain structure. This recrystallization
may include a continuous dynamic recrystallization mechanism
producing at least 50 percent high angle grain boundaries and an
intensity of basal (0002) texture not exceeding about 5. Moreover,
the strain rate ({acute over (.epsilon.)}) and the temperature (T)
preferably produce a Zener factor (Z) of greater than about
10.sup.9 s.sup.-1 as determined by the formula Z ={{acute over
(.epsilon.)}.times.exp(Q/RT)}.sup.-0.2, where Q is the activation
energy (135 kj mol.sup.-1), and R is the gas constant.
[0121] In at least one embodiment, the deformation strain rate is
in the range of approximately 0.1 to 50 s.sup.-1. The temperature
of the sheet bar 30 during deformation straining may be in the
range of approximately 150 C to 450 C and further, the deformation
strain may be at least 0.5. The deformation strain may further
plastically deform the sheet bar by predominately a slip mechanism
of the grain microstructure with less than 10% twinning and
substantially no shear banding.
[0122] In one further implementation of this deformation process,
the precursor is subjected to shaping of the material between a
pair of corresponding members having forming surfaces. The shape of
the forming surfaces imparts a large strain or strains, breaking
down the cast microstructure and producing new grain boundaries in
the precursor. Beginning with the sheet bar, initially formed so as
to have a fine grain structure of 10 .mu.m or less, this precursor
work piece is then shaped, for example, compressively into a
thinner flattened piece, between two forming members having
corresponding smooth forming surfaces. Preferably, the deformation
process is conducted at a warm temperature and the temperature of
the material is progressively lowered if any additionally passes
between the forming surfaces occurs.
[0123] Various schemes can be envisioned for deforming the sheet
bar 30. The sheet bar 30 may be passed through a rolling mill 200
having at least a first set of matching rolls 202 or a series of
matching rolls (not shown) or opposing pressing dies (not shown),
any of which may be heated. Also, the deformation process may be
performed separately from the formation of the sheet bar 30 or may
be integrated directly into the processing cell whereby the
apparatus 8 is provided with a transfer mechanism (which may be any
known variety and which is represented by line 204) to transfer the
sheet bar 30 from the mold 28 to a rolling or pressing mill
200.
[0124] In the illustrated rolling mill 200, the sheet bar 30 is
passed through at least one set 202 of opposed rollers 206. The
surfaces 208 of the rollers 206 are each designed to compressively
flatten the sheet bar 30. To achieve this, the rolls 206 may be
provided with smooth surfaces 208 that engage and compress the
sheet bar 30 as it passes between the rollers 206. The rolls 202 or
roll sets may be impressed towards each other by backup rolls 33
(shown in phantom) as is commonly known.
[0125] Referring to FIG. 2, alternatively, the opposed rollers may
be part of a flow forming arrangement 230. The flow forming
arrangement 230 may comprise of a first roll 232 having a first
shape 234 and/or a second shape 236. The work piece 30 may be
plastically deformed against the first roll 232 to form an ultra
fine grained shaped piece 238 by being spin formed and impressed
thereon by a second roll 240, which travels from a first end 242 to
a second end 244 of the first roll 232. Such a technique, generally
referred to as flow forming, may be used to produce, for example,
cylindrical shapes.
[0126] In at least one embodiment, lateral expansion of the sheet
bar is constrained. This may be achieved, referring back to FIG. 1,
by one of the rolls, for example, the lower roll 206, providing
raised lands 210 on the opposing ends of the roll 206. The raised
lands 210 are matched with the ends of the upper roll 206 in such a
way to constraint the sheet bar 30 from expanding laterally beyond
the lands 210 of the rolls 206. Moreover, by adjusting the lateral
position and constrain provided by the lands 210 of the rolls 206,
the thickness of the resulting sheetstock material 212 can be
controlled in relation to the original thickness of the sheet bar
30.
[0127] Alternatively, a Turks Head arrangement (not shown) may be
used to constrain edges of the sheet bar 30. A Turks Head
arrangement utilizes two or more pairs of rolls, one pair is
arranged vertically and the other pair horizontally. The vertical
rolls are spaced apart with the sheet bar 30 disposed between them
such that the edges of the sheet bar 30 contact the vertical rolls,
which limit the expansion, while the horizontal rolls compress and
flatten the sheet bar 30.
[0128] In at least one other embodiment, the work piece 30 passes
through the first set of rolls 202 and is received by a second set
of rolls (not shown), which may have a substantially smooth surface
similar to the surfaces 208 of the first set of rolls 202. The
second set of rolls further flattens the work piece 30 by imparting
a deformation strain. Additional set of rolls (not shown),
subsequent to the second set of rolls, may be used to further
flatten the work piece by imparting additional deformation strains.
In at least another embodiment, the work piece 30 is rotated
between progressive rolls sets, such as for example, rotating the
work piece 30 ninety degrees subsequent to passing the first set of
rolls 202, but prior to being received by the second set of
rolls.
[0129] Referring to FIG. 3, at least one other embodiment is
provided. In this embodiment a set of the sheet bars 30 are stacked
250, wherein the stack 250 is subjected to a deformation strain.
The stack 250 of sheet bars 30, which now form layers, may be
passed through the rolling mill 200. Again, the deformation process
may be performed separately from the formation of the sheet bar 30
or may be integrated directly into the processing cell whereby the
apparatus 8 is provided with a transfer mechanism 252, which may be
any known variety, such as for example, a robotic or rail-gantry
arrangement, to transfer the sheet bars 30 from the mold 28 to the
stack 250 and thereto the rolling or pressing mill 200.
[0130] In at least another embodiment, the stack 250 is plastically
deformed by the rolling or pressing mill 200, which bonds the sheet
bar layers 30 together. In one embodiment, the bonding process
occurs from friction strain welding of the sheet bar layers 30.
[0131] Moreover, reinforcing elements may be disposed between any
of the sheet bar layers 30 to provide a composite structure. For
example, reinforcing elements selected from the group consisting of
whiskers, graphite fibers, ceramic fibers, wires, wire mesh and
metal fibers may be disposed between two of more sheets bar layers
30 by any suitable automated process, such as by a robotic or
rail-gantry arrangement, or a manual process during stacking of the
layers 30. Then when the stack 250 is plastically deformed by the
rolling or pressing mill 200, the sheet bar layers 30 bond together
including the reinforcing elements, wherein the reinforcing
elements provide an enhanced load bearing function to the composite
sheet 212.
[0132] Alternatively, properties of the sheet 212 may be enhanced
by using selectively placed sheet bar layers 30 within the stack
250, wherein the sheet bar layers 30 are molded from respectively
different metal alloys having correspondingly different properties.
For example, the stack 250 may comprise a top and/or bottom sheet
bar layer 30 which is molded from a metal alloy material having
high corrosion resistance, such as for example, to a salt spray.
Moreover, the stack 250 may comprise other sheet bar layers 30
which are molded from metal alloy materials having higher yield
and/or ultimate strength and/or higher elongation. The stacking of
these layers 30 may be in a predetermined manner so as to adjust
the properties of the finished sheet 212 to a desired performance.
For example, a controller, or other device capable of performing
logical sequences, may be programmed accordingly and interface with
a robotic or rail-gantry arrangement which performs automated
stacking of these layers 30 in a specific order.
[0133] As an alternative scheme for deforming the sheet bar 30 or
the stack 250, at least one set of pressing plates (not shown) may
be used in place of the rolling mill 200. The plates may also be
provided with smooth, substantially flat surfaces or any other
suitable contour, which compressively deforms the sheet bar 30 or
stack 250.
[0134] Referring to FIG. 4, at least one other embodiment is
provided. The finished sheet may include layers formed from an SWP
process, which were stacked and subjected to deformation straining.
The SWP process involves the imparting of plastic deformation by a
combination of alternating tensile and compressive strains or
deformations. This step of deformation straining also permits
storage of dislocations within the microstructure, which leads to
the formation of new grain boundaries with high misorientation.
[0135] In one implementation of the SWP process, the precursor is
subjected to 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 imparts large strain,
breakdown the cast microstructure and producing 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 necessarily, 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.
[0136] Preferably. SWP is conducted at a warm temperature and the
deformation temperature of the material is progressively lowered
after each pass, for example, starting at 350.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.
[0137] As seen in FIG. 4, the sheet bar 30 is passed through a
first set 332 of opposed corrugated rolls 334. The surfaces of the
rolls 334 are each provided with corrugations 336 extending
circumferentially about the rolls 334. The corrugations 336 of each
roll 334 generally correspond with respect to one another such that
a ridge on one of the rolls 334 is received in a valley of the
opposing rolls 334. As the sheet bar 30 passes through the first
set 332 of rolls 334 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 331. Accordingly, the induced strains, tensile and thereafter
compressive, will be generally in the direction of the sine wave
shape itself.
[0138] Having been corrugated or worked by the first set 332 of
rolls 334, the worked sheet bar or work piece is passed to a second
set 340 of rolls 342. Upon encountering this second set 340 of
rolls 342, the work piece encounters corrugations 344 that are
oriented orthogonally, 90 degrees from the corrugations 336 of the
first set 332 of rolls 334. As such, the corrugations 344 are
oriented axially with respect to the rolls 342 and transverse with
regard to the direction of travel of the sheet bar 30. As with the
prior set 332 of rolls 334, the corrugations 344 of the second set
340 of rolls 342 are provided such that the ridge of a corrugation
on the upper roll 342 is received within the valley of a
corrugation 344 of the lower roll 342.
[0139] From the second set 340 of rollers 342, the worked sheet bar
is passed in the illustrated rolling mill 331 between a third set
348 of rolls 350 designed to flatten the worked sheet bar. The
rolls 350 may be provided with smooth surfaces 352 that engage and
compress the worked sheet bar as it passes between the rolls 350.
As the work piece is flattened, compressive strain is imparted to
the work piece to form an SWP sheet stock material 378. The rolls
340 and 350 are shown in more detail in FIGS. 5A and 5B.
[0140] As thus far described, SWP occurs generally according to the
process illustrated by the flowchart of FIG. 6. As shown therein,
SWP starts at box 366 wherein a sheet bar 30 is received and
subjected to corrugating in a lengthwise or parallel direction in
box 368. After lengthwise corrugating of the sheet bar 30, the work
piece undergoes transverse corrugation in box 370 and subsequently
is flattened as indicated in box 372. After being flattened in box
372, the lengthwise and transverse corrugating of the work piece
may be repeated as indicated by line 374. Optionally, as indicated
by phantom line 376, the work piece can undergo subsequent
lengthwise and transverse corrugation prior to being flattened in
box 372. However, it is believed to be preferable that flattening
according to box 372 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 372 and flat sheetstock material 378 is
outputted and the process ends in box 380.
[0141] As illustrated in FIG. 4, a stack 380 of SWP sheet stock
material 378 may be passed through the rolling mill 200 to form the
ultra-fine grain finished sheet 212. Again, the deformation
straining process may be performed separately from the formation of
the sheet bar 30 or may be integrated directly into the processing
cell whereby the apparatus 8 is provided with a transfer mechanisms
329, 382, which may be any known variety, such as for example, a
robotic or rail-gantry arrangement, to transfer the sheet bars 30
from the mold 28 to the SWP rolling process 331 to the stack 380
and thereto the rolling or pressing mill 200.
[0142] 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.
FIG. 7 schematically illustrates an additional manufacturing scheme
wherein the injection molding machine 10 of the first embodiment is
alternately replaced with an extrusion machine 400.
[0143] The extrusion machine 400 includes a barrel 402 within which
is located a screw 404. In that the other components of an
extrusion machine are well known to those skilled in the art,
additional discussion of the extrusion machine 400 is not provided
herein. Material is extruded from the extrusion machine 400 and
rapidly solidified between a pair of molds 406 such that a
continuous sheet of solid material is transferred from the
extrusion machine to the rolling mill 408. 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 408 in accordance with the present invention. The
rolling mill 408 illustrated in FIG. 7 is similar to either the
rolling mill 200 or 331 discussed in connection with the prior
embodiments. Accordingly, reference is hereby made thereto and
further discussion is not required.
[0144] With a 5.times.400.times.400 mm sheet bar 30 as the
precursor, the foregoing described processes can reduce the
thickness of the sheet to about 1 to 2 mm, wherein the final sheet
dimensions could be 1250.times.1250 mm. In at least one embodiment,
the stack of precursors is reduced in thickness, such that, the
ratio of the thickness of the stack to the thickness of the final
sheet is in the range of approximately 3:1 to 30:1. Additionally, a
ratio of a plan view (top view) area of the final sheet to a plan
view area of the stack is in the range of approximately 3:1 to
30:1.
[0145] When an integrated automated manufacturing cell, such as one
of those previously described, combines the rapid solidification of
metal injection molding with the deformation straining process 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.
Moreover, because of the fine grain microstructure of the sheet
bar, production yields of at least 80 percent are also
anticipated.
[0146] As would be surmised from the preceding discussion of the
invention, the as-molded grain size and .alpha. content of an
injection molded metal sheet bar is a favorable starting point to
attaining sub-micron grain size and low-anisotropy, typically not
exceeding 10 percent in yield strength, in the subsequently
plastically deformed sheet. It appears that the deformation
straining process, 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. More specifically, by minimizing twinning to less than
about 10 percent, deformation occurs substantially by slip in the
grain and grain boundaries without substantial shear banding,
resulting in a more ductile alloy.
[0147] 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 deformation straining 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.
[0148] 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.
[0149] 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.
[0150] Extensive deformation of injection molded material and the
like, to change grain boundary character, requires the extensive
deformation process that is accomplished in the present approach.
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.
[0151] 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, with
minimum 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, and extrusion molding. 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.) Once 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.
[0152] 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.
[0153] As an example, a commercial AZ31 B Mg alloy, notably not
semi-solid injection molded, 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.
[0154] Mechanical properties of AZ31 B 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).
TABLE-US-00002 TABLE 2 AZ31B Tensile Ultimate Normal Mg Alloy yield
tensile Anisotropy (room strength, strength, Elongation*,
e.sub.u**, e.sub.pu**, Ratio temperature) MPa MPa % % % (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)
[0155] 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.
[0156] For comparison, 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 90.degree. 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
[0157] 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%.
[0158] 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
[0159] 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. 8, the SWP material from
AZ91D 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 AZ91D alloy. Microstructures confirmed
that the coarse .beta. phase of the starting material had been
sub-divided and reprecipitated as nano-particles, some at grain
boudaries.
[0160] The feasibility of SPF of the SWP sheet has also been
demonstrated by the inventors. As FIG. 9 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.
[0161] In another example, AM60 magnesium alloy was semi-solid
injection molded in the commercial Thixomolding.RTM. machine to
produce 3.times.50.times.150 mm sheet bars. The bars were heated to
375.degree. C. and rolled in a mill in both stacked and un-stacked
arrangements. Table 5 provides the results of these tests.
TABLE-US-00005 TABLE 5 Number of Separation Number Grain Bars in
Percentage Force of RPM of Size the Stack Reduction (PSI) Passes
Rollers Bonding (mm) 1 58 81,000 1 45 -- 2-3 2 76 98,000 1 45
Excellent 1-2 5 85 140,000 1 Slow Excellent 1-2 1 42 64,000 1 45 --
2-3 3 81 114,000 1 45 Excellent 1-2
[0162] As illustrated in Table 5, in cases with 76 percent
reduction or greater, an ultra-fine grain microstructure was
achieved as well as excellent bonding between the sheet bar layers
of the stacked samples.
[0163] 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.
[0164] 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.
[0165] Deformation strain processing of suitable alloys should
reduce the cost of making thin sheet material by eliminating
multiple stages of rolling and annealing. This process changes the
grain boundary character and increases the ability to be formed by
warm forming or by superplastic deformation. If deformation strain
processing 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.
[0166] It is envisioned that the deformation strain processing 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.
[0167] 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.
* * * * *