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PERIODIC CELLULAR
MATERIALS:
MANUFACTURING
The applications of
periodic cellular metals (PCM) can be only realized if affordable
fabrication routes exist for the metal alloys of interest. Numerous
solid sheet forming, perforated sheet folding/drawing, wire assembly
and investment casting methods are emerging for the shaping of the
topologies discussed in the PERIODIC
CELLULAR MATERIALS: TOPOLOGY
section. Numerous ways for bonding these structures are also being
developed including laser and other micro-welding techniques for
steels, superalloys and many refractory metals, diffusion bonding for
titanium alloys and transient liquid phase processes such as brazing
for copper, stainless steel and some aluminum alloys.
Numerous approaches for
making metallic honeycomb structures have been developed in the
periodic cellular materials laboratory at the University of Virginia.
Hexagonal honeycombs can be fabricated by an expansion manufacturing
process, especially when the relative density is low,
 .
In this process thin metal sheet is first cut into panels and strip
bonded Figure 1. This process is referred to as the “honeycomb
before expansion” or HOBE method. This can be cut and stretched
perpendicular to the strip bonds to create a hexagonal structure. The
expansion process requires moderately high inter-sheet bond strengths
(sufficient to enable sheet stretching). For low density honeycombs
with very thin webs, the required bond strengths are readily
achievable with modern adhesives or by laser welding or diffusion
bonding processes. However, as the web (sheet) thickness to cell size
ratio increases (i.e. as the relative density increases) the force
needed to stretch the metal sheets eventually approaches the
inter-sheet bond fracture strength. The manufacture of higher
relative density hexagonal honeycombs then requires the use of other
manufacturing methods.
Figure 1: Schematic illustration of the honeycomb before
expansion (HOBE) method of fabricating hexagonal cores.
Another approach based
on a corrugation process is illustrated in Figure 2. In this
approach, a metal sheet is corrugated, and then stacked into a block.
The sheets are bonded by welding (or any suitable method) together
and the core sliced to the desired thickness and the corrugated
layers either adhesively bonded or welded to face sheets. Figure 2
shows the process for forming a hexagonal honeycomb core; however
this process may be used for numerous additional topologies including
square and triangular shaped cells
Figure 2: Schematic illustration of the corrugation method of fabricating
hexagonal cores.
In a third approach,
slotted metal strips can be assembled in the form of square and
triangular honeycombs, Fig. 3. Since no metal bending is required,
this slotted sheet process is also well suited for making honeycombs
from low ductility materials. The honeycomb cores are bonded by
welding/brazing (or any suitable method) together and bonded to face
sheets to form a sandwich structure. Even brittle composite or
ceramic honeycombs can in principle be made by this approach.
Figure 3: Schematic illustration of the slotted metal strip approach of
fabricating square and triangular honeycomb cores.
Prismatic topology
structures can be manufactured by sheet bending or progressive
rolling operations, Fig. 4, or by extrusion techniques. The
corrugation approach is often preferred for low relative density
structures made from alloys with high formability. Triangular,
flat-topped, square or sinusoidal corrugations can all be fabricated
by this approach. The flat topped structures have steeper webs than
their triangular counterparts making them a preferred topology for
many structural applications. The corrugated sheets can be stacked
either collinearly or in a cross ply manner and then bonded by
similar techniques to those used to make honeycombs.
Figure 4:
Schematic illustration of the corrugation method of fabricating
flat-topped prismatic cores.
Lattice truss topology
structures can be made by investment casting, perforated metal sheet
forming, wire/hollow tube lay-up or by snap fitting laser cut out
lattices. All but the investment casting approach require assembly
and bonding steps to create the cellular structure and for later
attaching it to face sheets.
Investment casting
begins with the creation of a wax or polymer pattern of the lattice
truss structure (and face sheets). The patterns for the lattices used
in flat panel structures can be made by injection molding. Those for
complex shaped objects can be made by rapid prototyping methods. The
truss and face sheet pattern is then attached to a system of liquid
metal gates, runners and risers that are made from a casting wax and
coated with a ceramic casting slurry. The pattern is removed and the
empty (negative) pattern filled with liquid metal, Fig. 5. After
solidification the ceramic is removed, the gates and runners are
removed, and the component is inspected to ensure that complete
liquid metal infiltration has occurred and that casting porosity has
not compromised structural integrity.
Figure 5: Investment casting method of fabricating lattice truss
sandwich structures.
In principle, the
investment casting approach can be used to fabricate complex,
non-planar shaped structures of significant size (1 – 5 m) and
weight (up to several hundred kilograms). Structures made from high
fluidity casting alloys such as Al-Si, Cu-Be
and some super alloys
have all been made this way. Most have relatively high core relative
densities (> 10%), a consequence of the difficulty of reliably
filling molds containing very small diameter, high aspect ratio
(truss) channels. Periodic cellular metals with lower relative
densities are better suited for fabrication from a wide variety of
structural alloys using sheet or wire forming methods.
The folding of a
perforated or expanded metal sheet provides a simple means to make
lattice trusses. A variety of die stamping, laser or water jet
cutting methods can be used to cut patterns into metal sheets. For
example, a tetrahedral lattice truss can be made by folding a
hexagonally perforated sheet in such a way that alternate nodes are
displaced in and out of the sheet plane as shown in Fig. 6. By
starting with a diamond perforation, a similar process can be used to
make a pyramidal lattice.
Figure 6: Punch and die forming of perforated sheet for fabricating lattice
truss cores.
There is considerable
waste material created during the perforation of the sheets used to
create low relative density lattices and this contributes
significantly to the cost of making cellular materials this way.
These costs can be greatly reduced by the use of either clever
folding techniques that more efficiently utilize the sheet material
or methods for creating perforation patterns that do not result in
material waste. Figure 7 shows an example of the use of metal
expansion techniques, followed by folding, that provides a means for
creating lattice truss topologies with little or no waste.
Figure 7: Folding of expanded metal sheets for fabricating lattice
truss cores.
The weaving and
braiding of metallic wires provides a simple, inexpensive means for
controlling the placement of metal trusses. It is applicable to any
alloy that can be drawn into wire. Plain weave (0/90°) fabrics
are the simplest to envision. The included angle (normally 90°)
can be modified after weaving by shearing the fabric and a wide range
of cell width to fiber diameter ratios are available. Cellular
structures are made from these metal textiles by simply stacking and
bonding layers of the fabric. The best structures are achieved by
aligning the layers so that the nodes of adjacent layers are in
contact, Fig. 8. Sandwich panels can be fabricated from these cores
with the wires oriented at any angle to the face sheet normal by a
transient liquid phase bonding process.
Figure 8: Folding of expanded metal sheets for fabricating lattice
truss cores.
Lattice truss
structures with similar topologies to textiles can be fabricated by a
wire or tube lay up process followed by transient liquid phase
bonding, node fusion welding or diffusion. Using a slotted tool to
control wire spacing and orientation it is a simple matter to lay
down collinear wires and to alternate the direction of successive
layers, Fig. 9. This procedure results in square or diamond lattice
truss topologies that can be machined and bonded to face sheets. The
approach has a number of attractions. Compared to the textile
approach, it is more straightforward to maintain the cell alignment
throughout the structure at low relative densities. Moreover, hollow
tubes can be used instead of solid wires and this enables very
low-density lattices to be achieved and truss compressive buckling
strengths to be increased by increasing the trusses moment of
inertia.
Figure 9: Wire and tube lay-up process
for fabricating lattice truss cores.
Many processes are
available to metallically bond honeycomb, prismatic and lattice truss
structures from the sub assemblies described above. For titanium
alloys diffusion bonding methods have been successfully developed.
For other materials, electrical resistance, laser, friction stir and
other common fusion welding methods can be used to assemble the
structures.
For many stainless
steels, super alloys, titanium, copper and some aluminum alloys,
brazing methods can be used to fabricate periodic cellular lattices.
Brazing usually involves coating (by dipping or spraying) the
materials to be bonded with a melting temperature suppressing alloy
dispersed in a binder/adhesive. For some materials, this can be
applied as a thin clad layer to the sheets prior to their assembly
into cellular structures. In other cases the brazing alloy is
suspended as a powder in a sticky polymer resin and is applied by
spraying. Numerous brazing alloy compositions have been developed for
the various engineering alloy systems of interest.
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