Forming of preconsolidated metal matrix composites

A method of hot forming bends or deflections in preconsolidated metal matrix composite panels comprised of unidirectional filaments of boron, borsic, alumina, or graphite in aluminum or titanium base metals without limitation of orientation of the filaments relative to axes of the formed bends or deflections.

This invention relates to the forming of metal matrix composites, and more 
specifically to the hot forming of preconsolidated metal matrix composite 
panels comprised of aluminum or titanium base metals containing boron, 
borsic, alumina, or graphite material filaments whereby bends or 
deflections can be formed in the panels without regard to the orientation 
of the filaments relative to the axes of bends or deflections formed in 
the panels. 
Preconsolidated metal matrix composites comprise a new family of sheet or 
panel materials that are of considerable interest to the technical and 
manufacturing or fabrication arts where strength-to-weight ratios are of 
great importance. It has become found and known in the prior art that 
these metal matrix composites with preconsolidated layers or plies of 
unidirectional filaments approximate the ultimate tensile strength and 
stiffness properties of high-strength steels, but weigh less than 
aluminum; they have very high fatigue and sonic fatigue strengths; and 
they are extremely resistant to fatigue crack growth as compared to the 
conventional sheet materials of steel, aluminum and titanium. These metal 
matrix composite materials are also without the problems of moisture 
absorption, temperature usage limitations, and electrical conductance 
usually encountered with fiber reinforced resin composite materials. 
Despite the valuable physical properties of the metal matrix composite 
panels recited above, serious limitation in application of these materials 
have heretofore resulted due to the inability of forming or shaping a 
panel member with the filaments arranged other than substantially parallel 
to the axis of a bend or deflection. In other words, it has been known 
that unless the filaments of each layer or ply of the composite were 
oriented in a direction substantially parallel to the axis of a 
unidirectional bend or deflection, the amount of bend or deflection was 
extremely limited to very large radius otherwise filaments would become 
ruptured and broken with a resultant loss or substantial reduction in the 
strength properties that would have been attained in the formed part had 
such filaments remained unbroken or unruptured. 
By practice of this invention, metal matrix composite can be formed with 
bends or deflections well beyond the heretofore limitations, and without 
concern or regard to the relative orientation of filaments in adjacent 
layers or plies. 
Accordingly, it is an object of this invention to provide a method or 
process for hot forming preconsolidated metal matrix composites by which 
bends or deflections of small radii can be formed without damage to the 
reinforcing filaments and maintaining the continuity thereof. 
A further object of this invention is to provide a method or process for 
hot forming preconsolidated metal matrix composites wherein the composite 
material can respond satisfactorily to hot forming without unacceptable or 
uncontrolled distortion. 
Another object of this invention is to provide a method or process for hot 
forming preconsolidated metal matrix composites with unlimited directional 
orientation of the filaments to the direction of bends or deflections and 
with unlimited orientation of filaments in adjacent layers or plies 
relative to each other. 
And yet another object of this invention is to provide hot formed 
preconsolidated metal matrix composite members or workpieces resulting 
from the above objects of the invention.

Generally stated, this invention involves heretofore unattainable forming 
of preconsolidated metal matrix composite sheets containing unidirectional 
filaments extending angularly relative to the axis of a bend or deflection 
by a method or process that can be generically identified as hot creep 
forming. Various combinations of materials to form such preconsolidated 
metal matrix composites are known or have been made, and constitute prior 
art that forms no part of this invention as this invention is only 
concerned with the hot forming of workpieces originating from 
preconsolidated metal matrix composites of sheet or panel form. 
The method or process of the invention involves the selective practice from 
the plurality of parameters comprising in the order of what is believed to 
be descending importance: an enclosure or enveloping containment of the 
workpiece during forming; forming temperature; die closing rate; die 
surface finish; closure angles on male die members; die clearance; dwell 
time after completion of die closure; and lubricant. Details of the 
parameters, along with the selectivity reasoning for the various 
parameters for forming the various types of preconsolidated metal matrix 
composites are discussed in more detail hereinafter. 
The prior art family of preconsolidated metal matrix composites, the 
manufacturing details of which form no part of this invention, includes 
those composites with typical cross-sections shown in FIGS. 1 and 2. In 
FIG. 1 the composite sheet or panel 10 is shown in cross-section and 
comprises a plurality of layers of unidirectional filaments 11 in a base 
metal 12. The filaments 11 of composite 10 may be of pure boron, boron on 
a graphite or tungsten substrate, borsic (boron with a coating of silicon 
carbide), or graphite, with the base metal 12 being of aluminum or 
titanium. Fabrication of composite 10, which forms no part of this 
invention, is accomplished by the lay-up of thin sheets or foils of the 
base metal 12 with layers of filaments 11 placed unidirectionally and 
interlamellarly between the sheets or foils of base metal 12 whereupon the 
lay-up is placed in a preconsolidated state by pressure diffusion bonding. 
Layers of filaments 11 can be oriented relative to each other in the 
lay-up in any desired manner to permit variation in load carrying and 
distribution properties of the composite 10 sheet or panel thereby 
attaining a variety of strength-weight efficiencies in composite 10. For 
example, the composite 10 depicted in FIG. 1 consists of four layers of 
filaments 11 with the alternate layers at a .+-.45.degree. to the plane of 
the drawing, or in other words, the filaments 11 of each layer are 
unidirectionally aligned at substantially 90.degree. or right-angles to 
the filaments 11 in adjacent layers. Also, as an exemplification only and 
forming no limitations to the invention, a typical four-ply composite 10 
of FIG. 1 would consist of four plies of approximately 5.6 mil diameter 
filaments (1 mil =0.001 inch), with approximately 1.8 mil thick sheets or 
foils of base metal between adjacent filaments layers and approximately 
3.5 mil thick face sheets on the outer surface of the outer filament 
layers which after pressure diffusion bonding results in a preconsolidated 
metal matrix composite sheet or panel of approximately 29 mils thick 
having approximately 45 to 49 percent of the volume as filaments. 
In FIG. 2, the composite sheet or panel 13 shown in cross-section comprises 
a plurality of unidirectional alumina filaments 14 of poly-crystalline 
Al.sub.2 O.sub.3 cast in an aluminum or titanium base 15. With the 
fabrication of composite 13 as a preconsolidated metal matrix by casting 
rather than pressure diffusion bonding, the filaments 14 are not in the 
layered order of filaments 11 in composite 10, but various filaments 14 
can be angularly oriented to other filaments 14 during the layout of 
filaments before casting the base metal 15 so as to achieve a variety of 
relative filament orientations in composite 13 in much the same manner 
attainable in composite 10. Also, it is to be noted that by casting 
composite 13, surface portions of some filaments 14 may be exposed to form 
a part of the surface of composite 13 rather than be completely surrounded 
by the base metal 15 as occurs in composite 10 with filaments 11 being 
substantially completely surrounded by base metal 12. 
Turning now to discussion of the parameters or steps of this invention as 
listed above, the one believed most pertinent to some of the composites to 
be formed is that of enclosing or enveloping of the panel or sheet 
workpiece during forming. As indicated above, it has been known in the 
prior art that bending or forming of preconsolidated metal matrix 
composites with unidirectional filaments extending other than 
substantially parallel to the axis of a bend or deflection has resulted in 
the fracture or breaking of the filaments with a resultant loss or 
reduction of the physical strength properties in the composite attained by 
the presence of the filaments. It has been found in the practice of this 
invention that the same difficulty of filament breakage or fracture occurs 
with some composites unless the outer, flat surfaces of the workpiece 
blank is enclosed or covered in a manner as shown in FIG. 3. Here there is 
shown a workpiece stock sheet or panel 16 of a composite 10 or 13 having 
an upper and lower titanium face sheet 17 connected to the outer, flat 
surfaces of workpiece stock 16 by a weld 18. This weld 18 extends 
completely around the periphery by seam welding or overlapping spot welds 
so as to completely surround the inner area portion of composite stock 16 
that will constitute the area of the formed composite workpiece after 
forming for after forming, the face sheets 17 and welds 18 will be removed 
by cutting or other appropriate removal to attain the formed workpiece of 
stock 16 with a portion of its edges removed. 
The face sheets 17 are of titanium in a commercially pure state or alloy; 
and may be in a work hardened or annealed condition, although an annealed 
condition is preferable due to the lesser springback properties thereof 
during cool down after forming that are reactive on the enclosed 
workpiece. Thickness of face sheets 17, while not critical, are preferably 
in the order of approximately 0.016 inch: thicker resulting in greater 
springback properties to be factored in during cool down, and thinner 
resulting in greater cost for the thinner sheet material. 
As stated above, enclosure or containment by face sheets 17 are required or 
important with forming only some of the composites by this invention, 
namely when forming preconsolidated metal matrix composites containing 
filaments that are coated such as in borsic composites where the boron 
filaments are coated with silicon carbide as a diffusion barrier at higher 
temperatures; composites containing filaments that are formed or made on a 
substrate such as boron filaments on a carbon substrate; and composites 
fabricated by casting such as composites 13 discussed above. Composites 
that do not meet at least one of the three immediately preceeding 
limitations may be formed by omitting the use of face sheets 17 and 
utilizing the following discussed parameters of this invention. 
The forming temperature is believed to be the second most pertinent factor 
for forming composites necessitating enclosure in face sheets 17, but of 
first importance to composites without such face sheet containment. 
Forming is accomplished with the workpieces and dies in the temperature 
range of from approximately 910.degree. F. to approximately 935.degree. 
F., and preferably at a temperature of 925.degree. F. with a tolerance of 
+10.degree. F. and -15.degree. F. Forming below this temperature range 
results in less plasticity of the metal matrix causing damage to the 
filaments during the forming, while forming above the temperature range 
presents a variety of problems of starting to get eutectic melting of 
aluminum base metals, degradation of filaments, and loss of alumina 
filament orientations in cast composites. 
After the composite and forming die are brought up to temperature, the die 
closure or forming strain rates can be varied from 5 to 15 mils per minute 
with the most preferable range being from 8 to 12 mils per minute. 
Principles involved with the die closure rate are the shallower the form, 
the greater the bend radii, and the smaller the orientation angle between 
the filaments and the bend axis, the greater the closure rate, and vice 
versa. Should the closure rate be too great, the workpiece material will 
fracture, and if too low results in time wasting forming inefficiencies. 
Also, it should be recognized that as the die members approach complete 
closure for a deep form or about small radii, it may be preferable to slow 
the closure rate as the composite workpiece approaches the maximum forming 
strain. 
The next order of importance is believed to be surface finish on portions 
of the forming die. With reference to FIG. 4, one embodiment of a forming 
die used in the practice of this invention comprises a male die member 19 
mounted to an upper press platen 20 and a female die member 21 mounted to 
a lower press platen 22. Closing alignment of die members 19 and 21 is 
maintained by any appropriate conventional means (not shown) so that as 
male die member 19 moves into enclosure with female die member 21, a 
composite workpiece extending across the space between die members 19 and 
21 at right angle to the direction of closure becomes formed to a 
configuration resulting from the die member shapes. During forming, the 
composite workpiece will have work forming or sliding contact or 
engagement with shoulders or corners 23 on male die member 19 and 
shoulders or corners 24 on female die member 21; the radii of shoulders 23 
and 24 forming the bend radii of the formed composite. The radii surfaces 
of shoulders 23 and 24 are die or draw polished to a finish of 8-16 RHR 
(Roughness Height Rating) or RMS (Root Mean Square) to minimize, if not 
eliminate, tool marks on the workpiece during forming by reducing pickup 
of the workpiece material by the tooling during forming. Also, when 
forming composites that are not contained by face sheets 17, it is 
preferable, though not mandatory or critical, to put a grinding finish on 
the bottom recess surface 25 of female die 21 to minimize, if not 
eliminate, surface defects on the composite surface that may appear from 
the pressure contact of the tooling surface with the relatively soft 
composite surface resulting from the forming temperature. Because of 
inherent springback qualities in preconsolidated metal matrix composites 
formed by this invention, the application of what is known in the prior 
art as draft angle or closed angle tooling is preferably incorporated in 
male die members as represented by angle 26 on male die member 19 in FIG. 
4. This angle 26 results in overforming of the composite workpieces to 
compensate for at least some of the inherent springback that occurs when 
the composite is removed from the tooling after forming. The range of 
angle 26 is from approximately 3.degree. to approximately 15.degree.; it 
being recognized a greater draft angle 26 should be utilized when any one 
or more of certain conditions apply or are present--namely, the thicker 
the composite the greater the draft angle, the greater the angulation of 
filament to the bend axis the greater the draft angle, the less the 
post-forming dwell time in the tooling (discussed below) the greater the 
draft angle, and the greater the resistance to bending by the filament 
material the greater the draft angle. 
Of the generic 3.degree.-15.degree. range for angle 26 discussed above for 
forming preconsolidated metal matrix composites, more preferable ranges 
for specific filament materials are approximately 3 to 5 degrees for boron 
filaments, approximately 5 to 10 degrees for borsic filaments, and 
approximately 7 to 15 degrees for both graphite and alumina filaments. 
The next item in the believed order of importance is that of forming die 
clearance or the spacing between the female die sidewalls and male die 
surfaces during die closure as represented by clearance dimension 27 in 
FIG. 4. This dimension 27 preferably ranges from approximately 1.3 to 
approximately 1.5 times the thickness of the composite being formed; the 
term composite thickness including the face sheets 17 when they are used. 
The importance of clearance provided by control of dimension 27 is that if 
there is too much clearance there is insufficient control, if any, of 
springback resulting in an inefficient forming of the workpiece blank, and 
too little clearance results in too much rubbing between the workpiece 
blank and tooling surfaces during forming which in turn causes damage to 
the composite filaments. 
Post forming dwell time is the last major factor of concern to the practice 
of this invention, and is the period following the complete closure of the 
die members with the formed workpiece and the tooling maintained at the 
above discussed forming temperature. This comprises a preferable period of 
from approximately 15 minutes to approximately 30 minutes, and an exact 
amount is both dependent and variable upon other factors. For example, the 
shallower the draw or greater the radius, the longer the dwell time; also, 
since there is less springback with longer dwell times, a smaller or 
lesser draft angle on tooling can be employed; or in other words, the 
shorter the dwell time, the more the material has to be overformed to 
reduce springback the more subject the workpiece is to filament damage. 
Dwell times longer than approximately 30 minutes are believed ineffective, 
resulting in only a waste of time and energy. 
The last item from the above listing involves the coating of the workpiece 
blank with a lubricant before forming. While this is not a critical or 
mandatory feature, it can be of assistance in forming deep draws by 
further minimizing rubbing contact between the blank and die surfaces, and 
hence, further minimization to the potential of filament damage or 
breakage as well as wear of die surfaces since the harder the die material 
the lesser the importance of a lubricant. A typical workpiece lubricant of 
the kind discussed above is one marketed under the trade name "Formkote 
T-50" by E/M Lubricants Inc., of North Hollywood, Calif. 
With reference now to FIG. 5, there is shown a longitudinal extending 
channel workpiece 28 depicting one section of layered cutaways 29 
representing the limitation of filament layer orientations in the prior 
art and a second section of layered cutaways 30 representing the greater 
strength-to-weight ratio filament layer orientations permitted by practice 
of this invention. Section 29 depicts in cutaway fashion three layers of 
filaments 31, 32, 33, with base metal sheets or foils 34 and 35 
respectively intermediate filament layers 31-32 and 32-33 and base metal 
face sheets 36 and 37. As indicated above, the depicted orientation of 
filament layers 31, 32, 33 so that they all extend unidirectionally 
parallel to the axis of any bend or deformation, whether formed by heat or 
creep forming, was a limitation of the prior art in forming 
preconsolidated metal matrix composites to avoid damage or breakage of the 
filaments, and hence loss of strength. Through practice of this invention, 
it has been found that such composites can be formed with filaments 
extending unidirectionally angulated relative to adjacent layers as well 
as to the bend or deformation axis without damage or breakage of filaments 
in a manner typified by cutaway section 30. Here there is shown three 
layers of filaments 38, 39, 40 with intermediate base metal sheets or 
foils 34, 35 and face sheets 36, 37; the orientation of filament layers 
38, 39 and 40 being such that filaments in layer 39 extend 
unidirectionally at 90.degree. to the filaments in layers 38 and 40, with 
the overall filament orientation of layers 38, 39, 40 being .+-.45.degree. 
to the axes of bends or deformations resulting in corners 41, 42 of 
workpiece 28. Thusly, not only is a composite formed according to the 
depiction shown in section 30 capable of carrying greater loads and have a 
greater strength-to-weight ratio than a composite according to section 29 
due to the angulation of filament layers to the bend axis, but the 
strength-to-weight ratio is even further enhanced by the relative 
angulation of filaments between adjacent layers thereof as shown in 
section 30. 
Referring now to FIG. 6, there is shown a flanged pan workpiece 43 formed 
by this invention which was considered completely unformable from a 
preconsolidated metal matrix composite by the prior art due to the 
inherent necessity of angulating filaments to a bend axis even with 
composites containing filament orientations as shown in section 29 of FIG. 
5. The cutaway section 44 of pan 43 is the same as section 30 of FIG. 5 
with filament layers 38, 39, 40 oriented in the same manner and located 
between base metal sheets 34, 35, 36, 37 as described above. 
In summary, it can be seen that by appropriate use and combination of 
tooling and process parameters as described above, hot or creep forming of 
preconsolidated metal matrix composites are accomplishable in accordance 
with the objectives of this invention recited above. 
While specific embodiments of the invention have been illustrated and 
described, it will be obvious to those skilled in the art that various 
changes and modifications may be made without departing from the invention 
and it is intended to cover in the appended claims all such modifications 
and equivalents that fall within the true spirit and scope of this 
invention.