Dissertation4

Investigation of
Cellular Materials
Author: Darren Syme
Supervisor: Neil Shearer
University: Edinburgh Napier University
Department: School of Engineering & the Built Environment
Course: BEng Hons Mechanical Engineering
Matriculation: 10007173

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Abstract
This report documents the investigation of manufacturing methods for cellular materials. At
the present moment there is a lack of understanding in the morphology of the structure
which has resulted in a lack of application. Through replicating manufacturing methods a
better understanding of the structure was obtained allowing applications to be determined.
Out of the methods identified two processes were replicated which provided two sets of
samples. Analysis was undergone through the use of microscopic and resilience tests. As
a result this project has delivered new concept ideas which could hypothetically influence
future engineering projects.

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Contents
Abstract..............................................................................................................................2
Contents.............................................................................................................................3
List of Terms/Abbreviations .............................................................................................5
List of Figures....................................................................................................................6
Acknowledgments.............................................................................................................8
1. Introduction....................................................................................................................9
2. Literature Review.........................................................................................................10
2.1. What is a cellular material? .....................................................................................10
2.2. Ceramics .................................................................................................................11
2.2.1. Replication technique........................................................................................12
2.2.2. Direct foaming...................................................................................................12
2.2.3. Gel casting........................................................................................................13
2.2.4 Hollow Spheres..................................................................................................13
2.3. Plastics....................................................................................................................14
2.3.1. Slabstock ..........................................................................................................15
2.3.2. Structural Moulding ...........................................................................................16
2.3.3. Extrusion...........................................................................................................16
2.4. Metals......................................................................................................................18
2.4.1. Liquid state Processing.....................................................................................19
2.4.2. Solid State Processing......................................................................................36
2.4.3. Deposition Processing ......................................................................................41
2.4.4. 3D Printing ........................................................................................................45
2.5. Applications of Cellular Materials ............................................................................46
2.5.1. Filters ................................................................................................................46
2.5.2. Heat Exchangers and Cooling Machines ..........................................................46
2.5.3. Noise Dampeners .............................................................................................47
2.5.4. Supports for Catalysts.......................................................................................47
2.5.5. Flame Arresters.................................................................................................47

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2.5.6. Impact Energy Absorbers..................................................................................48
2.5.7. Heat Sinks.........................................................................................................48
3. METHODOLOGY..........................................................................................................49
3.1. Limitations ...............................................................................................................49
3.1.1. Health and Safety..............................................................................................49
3.1.2. University Resources ........................................................................................50
3.2. Procedure................................................................................................................51
3.2.1. Fabrication Method of Metallic Closed Cellular Material ...................................51
3.2.2. 3D Printing ........................................................................................................52
4. Results..........................................................................................................................54
4.1. Fabrication Method of Metallic Closed Cellular Material..........................................54
4.2. 3D Printing...............................................................................................................58
5. Analysis and Discussion ............................................................................................60
5.1. Fabrication Method of Metallic Closed Cellular Material..........................................60
5.2. 3D Printing...............................................................................................................63
5.3. Possible Applications for the Tested Cellular Materials...........................................64
5.4. Critical Appraisal .....................................................................................................65
6. Conclusions and Further Work ..................................................................................66
6.1. Conclusions.............................................................................................................66
6.2. Further Work ...........................................................................................................68
6.2.1. Fabrication Method of Metallic Closed Cellular Material ...................................68
6.2.2. 3D Printing ........................................................................................................69
7. Appendices ..................................................................................................................70
8. References ...................................................................................................................73
9. Bibliography.................................................................................................................77

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List of Terms/Abbreviations
ABS Acrylonitrile Butadiene Styrene
CaOAl2O4 Calcium–Aluminium Oxide
CaO Calcium Oxide
Capillary Pressure Pressure generated between two immiscible fluids
CFC’s Chlorofluorocarbons
CMC Ceramic Matrix Composites
CO2 Carbon Dioxide
LFC Lost-Foam Casting
Morphology Internal design of foam
NaCl Sodium Chloride
PLA Polylactic Acid
Plasiticated Process of softening a plastic by heating and working
PPI Pore per Inch
RPM Revolutions per Minute
SDAS Secondary Dendrite Arm Spacing
SEM Scanning Electron Microscope
SLM Selective Metal Melting
Sn Tin
SrCO3 Strontium Carbonate
Syntactic Foam Materials made up of tiny hollow spheres embedded in a
surrounding material
TiH2 Titanium Hydride

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List of Figures
Figure 1, (1); open cell structure, (2); closed cell structure................................................10
Figure 2, Multiple Ceramic Processes...............................................................................13
Figure 3, Slabstock process ..............................................................................................15
Figure 4, Novel method using pre-saturated pellets ..........................................................17
Figure 5, Production method of direct foaming by blowing agents ....................................20
Figure 6, Effect of stirring time in correlation with viscosity................................................21
Figure 7, Temperature variances of blowing agent and metals .........................................22
Figure 8, Aluminium foams expanded ...............................................................................23
Figure 9, Gas injection process .........................................................................................24
Figure 10, Melt squeezing procedure ................................................................................26
Figure 11, Microstructures.................................................................................................27
Figure 12, Gasar process. .................................................................................................28
Figure 13, AL-Sn phase diagram.......................................................................................32
Figure 14, Resulting foams from being tested at various pressing temperatures ..............33
Figure 15, Environmental chamber....................................................................................35
Figure 16, SEM images of cell walls after sintering ...........................................................39
Figure 17, Before and after compression of hollow balls. ..................................................40
Figure 18, SEM of Aluminium foam coated in coppe.........................................................41
Figure 19, SEM image of nickel coated graphene.............................................................42
Figure 20, Schematic/SEM images of manufacturing technique .......................................44
Figure 21, SLM method .....................................................................................................45
Figure 22, Flame arrester ..................................................................................................48
Figure 23, AutoCAD model for 3D printing. .......................................................................53
Figure 24, Compacted pellets: (1); 40 minutes, (2); 1hour ................................................54
Figure 25, Sintered pellets: (1); 40 minutes, (2); 1hour. ....................................................54
Figure 26, Pellet (3) microscope image.............................................................................54
Figure 27, Pellet (3) after sintering. ...................................................................................54
Figure 28, SEM displaying the top of sintered pellet (3). ...................................................55
Figure 29, SEM showing exposed pores on the side of sintered pellet (3)........................55
Figure 30, Report 1 of elemental analysis. ........................................................................56
Figure 31, Report 2 of elemental analysis. ........................................................................57
Figure 32, 3D printed cellular material...............................................................................58
Figure 33, Displacement analysis......................................................................................58

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Figure 34, Resilience test ..................................................................................................59
Figure 35, Deposition rate vs solution temperature of nickel .............................................61

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Acknowledgments
First of i would like to give a massive thanks to my supervisor Dr Neil Shearer for his
assistance and guidance throughout the length of the project. Bill Campbell for 3D printing
my cellular material. Dr Callum Wilson for assistance with tests and further understanding
in the Advanced materials Centre. Thomas Ellam for letting me use his resillance machine
he created for his project.

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1. Introduction
With modern demands for new combinations of properties on the rise cellular materials are
one way in which companies are looking. A cellular material is a material consisting of a
porous structure throughout it, this means its density tends to be significantly lower than
that of a solid. These porous materials are often referred to as “foams”. In previous years
engineering has looked towards nature to conclude how it has done the job and tried to
integrate its key principles into the design problem, with this in mind Ashby, (2006) has
stated that;
‘When modern man builds large load bearing structures, he uses dense solids: steel,
concrete glass. When nature does the same, she generally uses cellular materials: cork,
wood, coral. There must be a good reason for it.’
With the above quote in mind and the desire for property combinations such as lightweight,
high stiffness and energy absorption, it has been seen that research on cellular materials
has seen huge finance being invested. This finance is mainly spent into their production
methods to refine the process and thus improve the “foam”. With huge cash investments
the materials commercial uses has been on the increase and are currently used by some
of the biggest aeronautical company’s around such as Boeing or NASA. Because of this
the following dissertation will look in depth about these production methods and try to
replicate a variety of them. The created “foams” will allow for testing to consider process
improvements and determine possible applications.
Numerous materials can be used to create cellular structures, they can be divided into
three category’s; Ceramics, Metals and Plastics. Although consideration has been taken
towards ceramic and plastic cellular materials the main study of this dissertation is to
investigate metal versions. Reasons being they have not been investigated as heavily and
researched compared to the others, yet hold the potential to solve a lot of material
demands.
The main aims of this research project are to identify possible methods of manufacturing
metallic foams, attempt to replicate several followed by determining ways of improving
them, and test the foams via resilience and microscopic testing. Finally applications for the
foams would be determined and possible ways of improving the process identified.

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2. Literature Review
2.1. What is a Cellular Material?
A cellular material according to Colombo, (2014) can be described as a material containing
a high porosity, this high porosity is caused by cells which fill its internal structure. Cells
are enclosed empty spaces which are surrounded by walls or edges. As the cells can vary
significantly it means the internal architecture of the cellular material can come in a variety
of designs, including; honeycombs, connected fibres, connected hollow spheres and
foams. Although numerous internal designs are achievable the most common are foams,
foams can be described as bubbles randomly orientated in three dimensional space within
the material. Because most methods within this dissertation are foams it will be common
for cellular materials to be referred to as a “foam”. A “foam” is recognised as a uniform
dispersion of a gaseous phase within a solid or liquid phase.
When a “foam” is created it will consist of either being open or closed cell, depending on
the application needed will depend on the type of structure required. An open cell foam
means its voids are connected via open pores whereas closed cell consists of the voids
being separated by solid walls. Figure 1 shows an open and closed cell material.
Figure 1, (1); open cell structure, (2); closed cell structure. (insulation.net, 2014)
Applications require diverse sets of cell structures, once an applications properties have
been determined a manufacturing process will be selected to alter the design of the
structures morphology; pore sizes, how many pores, distribution of the cells and how they
interconnect with one another. To change the morphology of the material production
methods are modified, these are often referred to as “controlling factors”. “Controlling
factors include various alterations to temperatures, pressure, pressing force, percentage of
foaming agent etc. These controlling factors will be spoken about in depth throughout each
metallic method whilst only briefly in plastics and ceramics.
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2.2. Ceramics
Ceramics are typically exploited for applications such as vases or providing heatproof
coatings, however unknown is there capability to create cellular materials. When selecting
a ceramic for a cellular material it tend to be ceramic matrix composites (CMC’s) that are
actually used. CMC’s are split into two categories, care should be taken when selecting
the material;
• Oxide: this consists of an oxide fibre and an oxide matrix such as Al2O3/Al2O3.
• Non-oxide: this contains a fibre and matrix of either carbon (C) or silicon carbide
(Sic), or a combination of both.
These cellular materials are used extensively in engines or as thermal insulation for
combustion burners due to their ability to cope with extremely high temperatures and allow
fluids to transfer through its pores. Because of this array of applications ceramic cellular
materials require numerous properties hence multiple processing techniques are required.
Properties required for such applications has been written about by David J. Green and P.
Colombo, (2013), also cited in the journal is how these CMC cellular materials will be
needed in the future to deal with technological advances;
‘Cellular ceramics display a rather unique combination of properties, such as low density,
low thermal conductivity, low dielectric constant, low thermal mass, high specific strength,
high permeability, high thermal shock resistance, high porosity, high specific surface area,
high wear resistance, high resistance to chemical corrosion and high tortuosity of flow
paths, making them indispensable for various engineering applications.’
There are currently four commercially used methods of producing cellular ceramic
materials, these are;
• Hollow spheres
• Replication technique
• Direct foaming
• Gel casting
Other production methods do exist for ceramic cellular materials, these are however small-
scale innovative methods which are only becoming possible due to new technology.

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2.2.1. Replication Technique
At the present time replication technique is the most commercially used process for the
manufacture of ceramic foams, this is likely due to the fact it originated in 1963. By being
developed that early it has allowing huge refinement/advances to occur, therefore it has
been able to dominate ceramic cellular material production. These advances have allowed
the process to become economical, consistent quality and mass production. One
unfortunate disadvantage is the restriction of only being able to create open cell foams.
To begin an open cell polymer precursor is selected, material choices include; polystyrene,
polyvinyl, latex and most commonly polyurethane. This precursor is then impregnated with
ceramic slurry (oxide or non-oxide), once it has been impregnated it is squeezed to
remove excess slurry and leave a homogenous distribution. This slurry is then left to dry
upon the precursor, additives are usually requiring to allow for a significant bonding, later it
is sintered to burn out the precursor. After sintering a foam will be left consisting of hollow
struts. Figure 2(b) shows a Scanning electron microscope (SEM) image of the foam
produced by this technique.
This sintered foam can then be machined to the desired dimensions, this however could
cause some damage or defects to the material. To prevent damage the precursor is often
dimensioned before slurry dipping, this ensures no damage will occur to the later applied
ceramic slurry.
2.2.2. Direct Foaming
With the use of a heated slurry it is possible to form bubbles which will rise and
accumulate which allows a foam to be created. To foam the bubbles a foaming agent can
be added to the mixture, this causes it to decompose under the influence of heat and
generate bubbles. Alternatively a mechanical stirrer such as an impeller can be used to
produce bubbles. Once the bubbles accumulate at the top of the melt conveyer belts can
be used to move it and allow solidification to occur. Once it has solidified a highly porous
cellular material will be left, both open and closed cell are possible. As this is one of the
primary methods of producing foams please refer to direct foaming (section 2.4.1) for an in
depth analysis on it. A foam produced by this method can be seen in Figure 2(a).

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2.2.3. Gel Casting
To begin a ceramic slurry is created, the slurry contains a gel-forming organic monomer
and a dispersant which when combined form a binder which is added to the ceramic
powder. To ensure a high quality foam is produced it is essential to reduce defects, by
placing the slurry in a vacuum it removes air thus preventing any bubbles forming. As this
process relies on chemical reactions a catalyst is added to form cross-linked molecules
which will trap any water thus creating an elastic gel. So after the catalyst has been added
it is poured into a cast and heated in a curing oven to cause the chemical reactions. It is
then unmoulded and sintered to remove the binder thus leaving a ceramic foam. There are
advantages that come from gel casting which other ceramic methods don’t have, the main
ones being; extremely low defect numbers and high strength. This method is similar to that
of reaction sintering (section 2.4.2) in the sense it requires chemical reactions to work.
When it is left to dry it will shrink, Tulliani, (2013) found it shrunk between 18% and 30%
which is significant so shrinkage needs be taken into account.
2.2.4 Hollow Spheres
This technique involves the use of filling a mould with hollow cylindrical spheres,
compressed, then sintered. After compression a slurry will tend to be poured over to join
the beads during the sintering process. Sintering allows for densities of up to 10%
(compared to theoretical density). It is possible to fill the mould with different shaped
space holders if desired. Again this method is used for producing metallic versions so
further information can be seen in hollow spheres (section 2.4.2).
Figure 2. (a) Shows direct foaming, (b) shows replication technique, (c) shows a reticulated foam
(no method specialised). (Colombo, 2014)

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2.3. Plastics
Plastic cellular materials are by far the most commercially produced out of the 3 materials
choices, applications vary from insulation to food packaging. Reasons for the widespread
use is because of numerous properties that can be achieved, these include; low density
allowing for significant weight reductions, flexible or rigid and low heat transfer. One other
advantage is their low cost, this has been allowed by the sheer numbers they are
produced in. The reason for these large numbers has all come down to refinement
occurring in manufacturing techniques. Plastics can be divided into either thermosets or
thermoplastics and can be further divided into rigid or flexible;
• Thermosetting plastic; a petrochemical material that once heated and left to cool
can’t be reheated and melted again to a different shape.
• Thermoplastic plastic; a polymer which when heated above a specific heat will
become mouldable. It is referred to as having “memory” as it will return to its original
shape when left heated.
One large issue faced with plastic foams was the banning of chlorofluorocarbons (CFC’s),
commonly known for being in aerosols their other large role was acting as a blowing agent
in plastics. By banning CFC’s for corroding the ozone layer a significant amount of plastic
foams stopped getting produced for a period of time, the only alternative was to use a
different blowing agents which were classed as environmentally friendly. With the issue of
becoming environmentally friendly it is becoming ever important that the plastics can be
recycled once used. Due to a thermosets molecules being heavily cross-linked it is less
likely they will be used in future productions of plastic foams due to being harder to
recycle. So without the use of CFC’s and the reduction in thermosets there are three main
ways to produce cellular materials, these include;
• Slabstock
• Moulding
• Extrusion
All these methods allow for high production levels which have allowed for them to become
popular due to the cost per piece being extremely low.

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2.3.1. Slabstock
Slabstock is a process which relies upon chemical reactions to occur, the chemical
reaction is caused between isocyanates and polyols. So by combining isocynates and
polyols a Poly (urethane) will be created, characteristics of the resulting foam are
dependent upon the ratio of the two chemicals. To create the foam both chemicals are
heated and combined in a mixing head, this is then poured in a trough and eventually onto
a conveyor where it will foam large slabs of material. As large “slabs” of foam are created it
has acquired the named slabstock. Figure 3 displays the process.
Figure 3, Slabstock process; (1) raw material, (2) mixing head, (3) Trough, (4) Creaming, (5)
operators platform (6) bottom paper feed, (7) fall plates, (8) horizontal conveyor. (Polygrow, 2014)
Unfortunately the process is significantly harder than just combining polyols and
isocyantes to create the foam. As the raw materials are sent down tubes there are
combined in a mixing head, it is here where a rotor is used to distribute the two chemical
homogenously throughout a liquid. Along with mixing the materials the rotor also inserts
bubbles into the mixture, these will be left as pores in the final structure so care must be
taken to mix at the appropriate speed. Pores are also created by the carbon dioxide being
diffused through the liquid by the polyols, this means both the rotor and carbon dioxide
pores need to be accounted for when designing. Although there are mechanical inputs the
manufacturer has control over it is up to shear chemistry and physics to what the foam will
appear like after leaving the mixing head.
According to testing, (unknown) one of the main problems with creating a consistent foam
is keeping a constant temperature of the raw materials. As chemical reactions vary
significantly with temperature it is crucial to ensure the materials used are in the desired

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temperature range. This requires the materials to be stored in a climate controlled
environment to prevent any inadequate batches occurring.
2.3.2. Structural Moulding
Structural moulding is a pressure injection process, molten plastic is combined with a
blowing agent or highly pressurized gas inside a mould. This process creates a foam or a
pattern resembling honeycomb depending upon the controlling factors used. In general it
is similar to that of injection moulding with the exceptions of it being moulded at low
pressure and a blowing agent added. Thankfully the blowing agent does not start the
foaming process until it reaches the mould, as a result the molten plastic flows further
which is the reason for only needing low pressure. As low pressure is used capital is saved
in the process with both production (less forces required for clamping) and machine costs
(lower quality materials can be used).
When manufacturing care has to be taken into designing the mould, typically it contains
runners (thick walled sections) with a narrower centrepiece. Unfortunately the final product
will have shrunk by a small percentage of the mould once removed, this requires
designers to make the mould slightly larger. The final product consists of thin walls with an
internal cellular structure, reason being the plastic solidifies flowing across the mould.
Commonly used for this process is high density polyethylene (HDPE) however plastics
have a tendency for a swirling finish to occur upon the final surface of its walls. Aluminium
can also be used and the surface swirling does not occur.
2.3.3. Extrusion
The extrusion process is known for creating uniform solids in a singular direction, however
with modifications and the addition of a blowing agent such as carbon dioxide (CO2) it is
possible to create microcellular materials. This process has three key requirements;
1. “Plasticate” the polymer
2. Create a polymer-gas solution
3. Promote high cell nucleation
Assuming a single screw extruder was used the process begins by placing polymer pellets
into a hopper where they will be heated and crushed up. Once crushed they reach the
compression screw where the polymer is “plasticated” and put under high pressure. Static

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mixers are used at some point along the compression screw, location is dependent upon
the model, and it is here where the polymer and blowing agents are uniformly distributed.
After the compression screw a custom designed nozzle is, this creates a huge pressure
difference which causes a thermodynamic instability that in turn nucleates millions on
minute bubbles, known as diffusion zone. More complex machines are fitted with a second
diffusion zone, this allows for better dispersion and finer bubbles. To finish it is pushed
through a die where it can be cut to the required length.
There are several ways to insert a blowing agent into this process;
• By adding it in the chopper it will mix between the crushed pellets, complex screw
designs are required to mix the two and allow uniform distribution as the static
mixers will not be enough.
• By adding a second nozzle to the compression chamber which will insert an inert
gas such as CO2. This process requires a large amount of modifications to the
machine, the biggest one being high pressure and temperature seals around the
entry point, failure to do so would result in complete failure.
• Alternatively a second screw can be added which will enter supercritical gas.
• Kumar, (2004) showed a very intriguing and novel method, it suggested the use of
pre-saturated pellets in CO2. By. By pre-saturating the polymer pellets in CO2 they
would absorb it, it was said the pellets would become “charged” as the CO2 would
fill all the micro-imperfections. So once it was heated and crushed in the hopper
nucleation would begin. Advantages from this include no need for diffusion zones
and the material would be of a consistent density due to a known level of saturation.
This can be seen in Figure 4.
Figure 4, Novel method using pre-saturated pellets. (kumar, 2004)

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2.4. Metals
As previously mentioned in section 1 metals are the primary focus of this dissertation.
Several reasons exist as to why metallic cellular materials are not being used extensively
as they can be, the majority of them surround problems faced when manufacturing.
Manufacturing cellular materials demands extensive controlling factors at all times,
unfortunately this generates cost which has never allowed them to be created to a cost
effective price when compared solid components. Other problems faced tend to related to
recycling; firstly there is the problem with trying to recover as much material as possible
before disposing of, and secondly governments are pressing on reducing waste generation
whilst ensure hazardous waste is disposed of appropriately. These problems have all
contributed to an ever increasing overall cost which sadly means there is a real struggle of
competing with other methods.
There are currently three groups of methods in which the cellular materials can be created;
• Liquid State
• Solid State
• Deposition
These three methods allow a substantial amount of cellular materials structures to be
made, most of which supplying a different combination of properties. The benefit of having
so many production methods allows the use of so many metals to be used, these include;
nickel, aluminium, titanium and even alloys. The only concern is that the appropriate
manufacturing method has been selected to go with the material. It should be noted that
liquid and solid state have a large amount of sub-productions, admittedly some are
completely novel ideas which will most likely not become mass produced however there
are several which have high potential with extensive research.

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2.4.1. Liquid State Processing
The first group relies upon molten metal to be used for the creation of a cellular structure.
The molten metal can be changed into a cellular material by either;
• Using an indirect method such as using a polymer foam.
• By directly foaming the molten liquid.
• Melt powder pellets containing a blowing agent, such as the pre-saturated pellets
mentioned in extrusion (section 2.3.3).
• Or by casting it around space holders, these can later be removed via some sort of
treatment which in turn leaves a cellular material.
Direct Foaming
Under controlled circumstances it is possible to foam metal melts by creating bubbles
throughout it. This process requires a large amount of control as bubbles formed in the
melt tend to rise to the top quickly due to the metals viscosity. However when the viscosity
is increased the speed is reduced as the melt becomes “thicker” thus it requires
increasing. Viscosity can be increased by incorporating alloying elements or ceramic
powders to the melt which act as stabilisers in the metallic melt.
This method was experimented with throughout the 1960’s, however it did not take off and
no real life applications seemed to need it at the time. There is no real evidence about the
control process used at the time hence the possible reasons for it not being used include;
• Inconsistent quality and production levels.
• Too expensive to produce compare to other materials on the market.
• No demand for metallic foams.
In the present however there is an ever increasing level of technology allowing quality’s to
be increased, cost reduced allowing foam to be on the comeback. There are two main
processes used for direct foaming;
1. Direct foaming by blowing agents.
2. Direct foaming by gas injection.

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Direct Foaming by Blowing Agents
The first method involves adding a blowing agent to a metallic melt, once in the melt it
begins to decompose due to the heat of the melt. This reaction causes the blowing agent
to release gas thus start the foaming process. Figure 5 displays the method. In industry
one common material combination for this method is introducing Calcium to an aluminium
melt at approximately 680oC. Once the calcium has been added the melt is stirred for
several minutes in which its viscosity continuously increases by a factor of up to five, this
forms calcium oxide (CaO), calcium–aluminium oxide (CaAl2O4) or Al4Ca which thickens
the melt. This means stirring to be used as a controlling factor; by varying the stirring time
the viscosity will be different, this can be seen in Figure 6 along with the effect of different
weight percentages of calcium metal. Once the desired viscosity has been reached a
blowing agent is added to the mixture which will releases gas thus forming pores, a
popular choice is titanium hydride (TiH2).
Matijasevic, (2005) expresses;
‘After partial melting the gas released by the blowing agent then leads to the formation of
spherical pores provided that the liquid fraction is sufficiently high’.
As the gas is being released it is essential to keep a constant pressure so that the
expansion rate remains stable. Once fully expanded it is then left to cool and can be cut
into desired sizes. Foams produced by this method tend to create the most homogeneous
cell distribution and hence the reason why it is one of the most popular. In industry it is
referred as to “Alporas” foam.
Figure 5, Production method of direct foaming by blowing agents. (The minerals, 2014)

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Arguably the most important factor to consider with this process is that there is only a
small temperature variation between the metal used and blowing agent. Matijasevic,
(2005) investigated the situation with how foam could be improved by the tailoring of a
blowing agent. If the blowing agent’s melting point is significantly lower than that of metal it
will cause a heterogeneous pore structure which consists of crack like pores, this is
because gas is being released into a solid state metal. Again problems occur if the blowing
agent’s melting point is substantially higher than that of the metals, viscosity will decrease
when the metal gets heated so the formation of bubbles will not be consistent and a very
poorly formed foam will be created. Also the metal will begin to melt.
Figure 6, Effect of stirring time in correlation with viscosity with various calcium added.
(J.Banhart, 2001)
The journal Matijasevic, (2005) states that at first untreated TiH2 was experimented with
aluminium alloys, however TiH2 has a melting point of 400oC whereas the aluminium alloys
was 525oC, and this resulted in a poor quality foam. The aluminium alloy was then
replaced with two alloys;
• AISi7; Showed better results however not good enough to be commercially
produced.
• ALSi6Cu4; Out of the metals attempted this produced the best pore structure
however it still had a higher melting point than the TiH2.
It became apparent that untreated TiH2 did not match the characteristics for melting as any
existing alloys leading to further development of the substance. With further advances in
technology it was possible to pre-treat the TiH2 by using oxidising conditions and thus an
acceptable powder was formed by using the ALSi6Cu4 alloy. Another problem is TiH2 in
powder form and precursor form showed slightly different temperature variations.

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Displayed in Figure 7 is the temperature variations as well as the melting temperatures of
the alloys and blowing agents.
Figure 7, Temperature variances of blowing agent and metals. (Matijasevic, 2005)
These advances have allowed the technique to evolve and produce significantly better
cellular materials. Figure 8 expresses the improvement of using the tailored TiH2. Metalek,
(unknown) are a company which produce foam via this method, they manufacture it for
numerous applications but most interesting is defence vehicles. This foam works
exceptionally well for vehicles likely to get hit by explosions due to its array of properties;
Energy absorption, multi-hit capabilities, isotropic, lightweight, recyclable, improves
structural integrity and noise dampening. One of the most beneficial properties in relation
to this application is its surprising ability to take multi-hits, without foams the humans within
would be more likely to get injured.
Direct Foaming by blowing agents is expensive compared to the bulk of other cellular
manufacturing methods. There are novel ideas still being created for alternatives to the
method, some of the more interesting methods of discovering new material properties
includes;
• Recycled egg shells are used as the calcium additive to increase the viscosity, this
could potentially be used in the future to increase recycling demands.

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• Replacing calcium from the process completely and blowing bubbling air through
the alloy. This however means heat cycles are very hard to obtain and as a result
end up more expensive to run than adding the calcium.
• ‘The blowing agent is added to a liquid alloy at a temperature just above its solidus
temperature but below the decomposition temperature of the blowing agent. After
intense stirring the metal is allowed to solidify in a mould of the desired shape. The
actual foaming takes place in a second step when the composite is heated to a
temperature above the decomposition temperature of the blowing agent. The
evolving gas then produces bubbles and the volume increases.’ J.Banhart, (2001).
Figure 8, Aluminium foams expanded to approximately 2, 2.5 and 4 times the height of the
precursor). Left column is untreated TiH2, right column is treated TiH2. (Matijasevic, 2005)

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Direct Foaming by Gas Injection
Alternatively metal foams can be produced by creating bubbles throughout the melt using
an inert gas as the foaming agent. An impeller consisting of a hollow centre is placed
inside an aluminium melt, this allows gas to be inserted into the mix by injecting through
the hollow inside, and gases are typically argon, air or nitrogen. When the impeller is at full
rotation the gas is injected which as a result creates fine gas bubbles which once created
rise to the surface of the aluminium melt. After a small period of time bubbles collect at the
top and create a dry liquid foam which is moved with the use of a conveyer belt. During its
movement on the conveyor belt it begins to solidify, at this stage is possible to compress
the foam to the desired thickness. As with direct foaming by blowing agents (section 2.4.1)
is it essential for reinforcing particles to be used, the production method can be seen in
Figure 9.
Figure 9, Gas injection process. (J.Banhart, 2001)
There are several factors that need to be investigated before the process should be used,
these include; viscosity, dispersion of reinforcing particles, particle wetting and drainage.
As a homogeneous melt is needed to create a high level of quality, reinforcing particles
should be evenly distributed throughout, several choices are available; silicon carbide,
magnesium oxide or aluminium oxide. The method of doing so could be done by pre-
saturating the metal before being melted or adding additional mixers into the aluminium
melt to ensure maximum dispersion of the particles.
As reinforcing particles must raise the viscosity to a desired level as such that a certain
level of porosity occurs, it is crucial that the viscosity achieved is accurate. Failure to do so
will cause either to many or too few bubbles rising to the top, resulting in a poor foam.

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Particles wetting is also a large factor which needs considering, a good wetting prevents
the bubbles from being stripped away from the gas whilst ensuring they are stabilised.
However there is also the concern that if the wetting is to high stabilisation will not occur
and again causing problems with the produced foam.
Drainage has been investigated in the book; V. Gergely, T.W. Clyne, (2004), although it
wasn’t specific for one method it will have a big role in this process, due to it solidifying
along the length of the conveyor belt. It found that small cells, high porosity and the higher
the number of cell faces inhibited drainage. As the drainage occurs a capillarity pressure is
developed in the vertical gradient, this can cause some significant drainage effects;
‘This gradient partially counter-balances gravity-driven flow and often has a significant
effect. In fact, it can completely suppress drainage under some conditions. A high
capillarity pressure gradient is favoured by higher porosity, smaller cells and shorter
specimens.’ V. Gergely, T.W. Clyne, (2004).
To try and prevent a capillarity pressure occurring and preventing drainage there are
several methods which should be looked into to minimise drainage;
1. By increasing the speed at which bubbles are being formed, more gas would be
injected and propeller rotates at a larger rotations per minute (RPM).
2. Increase bubble stabilisation to prevent ruptures thus ensuring finer cells to be
created. This can be achieved by an appropriate wetting ratio of the bubbles.
Using this method it is possible to create porosities in the range of approximately 80-98%
showing it can be made extremely light weight. The pores using this process are that of a
polyhedral shape, this is due to the drag the bubbles occur as they initially rise, they begin
as spheres. There are alternatives materials to aluminium that have been tried and tested,
these include; magnesium or zinc and alloys which consist of one the materials mentioned.
The advantage of this process is that large continuous foams can be created allowing it to
be manufactured at a relatively low price, however as this will require cutting the cells will
be open and exposed thus creating a weak point. One idea of preventing the exposed
cells is to place a mould at the end of conveyor which will change upon being filled.

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Melt Squeezing Procedure
Melt squeezing is a novel process that both Roudini, (2012) and Ramin Jamshidi-Alashti ,
Mehdi Kaskani , Behzad Niroumand, (2014) investigated. It is a process that creates
open-cell foams based on liquid state, it is a novel yet inexpensive process which on paper
could solve the problem with the costing’s of foams. A metallic melt is created and a space
holder (has to be possible to be leached) is added, then stirred thoroughly to achieve the
best possible distribution throughout the melt. Space-holders should be preheated to
ensure solidification doesn’t occur within the melt and allows for easier flow throughout.
With the use of a perforated piston excess metal can be removed, it is essential that the
size of the perforations are smaller than that of the space-holder to prevent them escaping.
By applying a large pressure it increase the chances that the space-holders will
interconnect which will in later stages allow for their removal. Once excess metal was
removed from the piston the melt is left to solidify followed by either heat treatment or
leaching in water to remove the space-holder (depends on the type). If a syntactic foam is
desired then the space-holder would left untouched. This process can be seen in Figure
10, it should be noted that constant heating is required to keep the metal molten unless it
will solidify.
Figure 10, Melt squeezing procedure. (Roudini, 2012)
Both journals used a salt as a space-holder and aluminium as the melt, it is unknown
whether our material choices would work however in theory there is no reason why not.
They also came to the same conclusion that the majority of mass from the cellular material
being in the walls, approximately 75% by mass. The main difference was Ramin
Jamshidi-Alashti , Mehdi Kaskani , Behzad Niroumand, (2014) looked into how the
microstructure changes with pressure/temperature whereas Roudini, (2012) investigated

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how stresses change with pressure increase. When the microstructure was studied there
was evidence of changes in grain size, secondary dendrite arm spacing (SDAS) and grain
roundness. These microstructures can be seen in Figure 11, Appendix 1 states which
pressure/temperature combinations were used.
Figure 11, Microstructures; (a) FA356, (b) FSS12, (c) FSS25, (d) FSS35, (e) B0, (f) B0+Salt, (g)
BSS25, (h) BSS25 + salt and (i) FAl–3%Si. (Ramin Jamshidi-Alashti , Mehdi Kaskani , Behzad
Niroumand, 2014).
Roudini, (2012) investigated the effects of varying the piston load upon the melt, this was
to find how porosity, densification strain, plateau stress and stress were affected, Table 1
shows the findings. The key findings were as load increases, porosity increases while both
plateau stress and stress decreases.
Table 1, effect of varying piston load. (Roudini, 2012)
Piston load (MPa) Manual 2 4 7 10
Porosity (%) 67 69 79 81 85
Densification strain (εD) 59 43 62 51 N/A
Plateau stress (MPa) 17.7 8.08 5.67 2.58 N/A
σ (MPa) 4.7 2.17 1.36 0.57 N/A

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“Gasars”
Another recent production method has been developed to create a porous structure
consisting of long cylindrical pores which tend to be aligned in one direction. This method
is known as “Gasars”. By melting metal in a highly pressurised hydrogen atmosphere an
invariant reaction occurs creating a heterogeneous two stage system, this consists of both
a solid and gas combination. By removing some of the heat the solidification process
begins which in turn;
‘Gas pores and the solid metal grow in couples and finally form a regular porous structure
in which the long gas pores are aligned parallel to the solidification direction.’ Yuan,
(unknown).
This production method can be seen in Figure 12.
Figure 12, Gasar process (Yuan, unknown).
Because of its porous layout it is thought to look like a lotus plants roots. Porosity of the
structure is dependent upon;
• Gas pressure
• Melt temperature
• Hydrogen content
“Gasars” have been found to have enhanced properties compared to other porous metals
by sintering or foaming thus possible application ideas are expected in the near future.
Two of its key attributes are strength in compression and tension.

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Casting Methods
Lost-Foam Casting
Lost-Foam casting (LFC) starts by dipping a polymer foam with a slurry consisting of a
heat resistant material such as calcium carbonate or phenolic resin. The polymer foam
should be pre-fabricated to the shape of the desired foam. It should be noted that if the
polymer foam is open cell then it will be changed to a closed cell by the slurry and vice
versa for a closed cell. Once the slurry has cooled it can be placed into a furnace to burn
away the polymer foam allowing a metallic melt to be poured into the open pores which will
replicate the structure of the original polymer foam.
Although simple gravity casting can be used it is quite common for the metallic melt to
solidify at a quick rate due and block the pores. To prevent this heat can be applied to the
mould to decrease the solidification rate along with applying a force via a piston to the
metallic melt, the piston can also be heated. Once set the slurry material can be removed
by such methods as applying pressurised water, leaving a replica of the starting polymer
foam. As the pressurised water is sprayed directly at the foam it is hard to prevent any
damage from occurring on the external structure meaning extra time has to be spent trying
to prevent this with care being taken.
This process is rather expensive compared to most others however with increasing
process refinement costs are on the decrease. However to justify the expense complex
shapes can be produced by fabricating of the polymer foam. Reasons for the high costs
include;
• Length of time required dealing with cooling.
• Time required to “burn out” precursor.
• Time required removing slurry mix.
• Need to buy/make a polymer foam to build on.
• Cost of slurry.
Porosities are possible of up to 97% and typical pores per inch (PPI) varying from between
5-40. As porosity/PPI are predetermined by the original polymer precursor it is in essence
able to reach the porosity of any plastic cellular material. It should be noted that as
porosity increases struts of the cellular material will become thinner, this means additional
care has to be taken when removing the ceramic slurry.

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Casting Around Space Holder Materials
The alternative form of casting involves using a material as a space holder which usually
consists of being hollow spheres or granules. A syntactic foam is often created using this
process as the hollow spheres tend to be left in, however depending on the material it can
be possible to leach them out. When leaching it is essential that there is a high number of
granules which interconnect so the leaching can occur throughout the material. Methods of
leaching include acids or heat treatment techniques.
One problem with this technique is premature solidification, this occurs when the melt is
poured on top of the granules and prevents the melt reaching the bottom of the mould. To
prevent this the space holders and mould can be heated to increase solidification time of
the melt. Another crucial step is to ensure the hollow spheres are completely dry, if not
water will evaporate leaving a void within the structure.
Usually there is a slight variance/error with pore sizes using most methods due to having
to control decomposition of a foaming agent, however pore size depends solely upon
space holders allowing complete control over the process. Two other but not as significant
advantages include the ability to design a mould can which will accompany complex
shapes and pore uniformity can be varied but altering the location of the space holders.
One disadvantage is that previously mentioned space-holders have to interconnect, this
requires they take up a larger portion of the foam, J.Banhart, (2001) states that because of
this;
‘The maximum porosities which can be achieved using space holders are limited to values
below 80%, whereas direct foaming allow for porosities up to 98%’.
There are quite a variety of material options for this technique, materials for the matrix
include; aluminium, magnesium, zinc or lead. While the filler can consist of either organic
or inorganic options which consist of; Polymer spheres, sand pellets which can be
removed via heat treatment as it will dissolve the bonding agent. There is also sodium
chloride (NaCl) which has numerous advantages such as low cost, fast dissolution in water
with a reduced corrosive attack on the metal during dissolution and it’s free of toxicity.

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Powder Compact Melting Technique
To start this process a blowing agent and metal powder need to be mixed and compacted
into a dense pellet (any shape is acceptable). The pellet can be compacted using any
method in theory as long as the blowing agent is ingrained within the metal powder.
Although the compaction method tends to be determined by the foams shape, extrusion is
the most commonly used due to it being the most economical. It is crucial to ensure there
are minimal defects or residual porosity on the pellet to prevent imperfections occurring in
final product.
As mentioned with direct foaming by blowing agents (section 2.4.1) it is vital to ensure only
a small temperature difference occurs between the blowing agent and metal powder!
Because of this blowing agents are pre-treated and a select few metal powders chosen to
use in this process. Once compacted the pellet is heated to the melting point of the matrix
where the blowing agent will begin to decompose causing the material to expand thus
leaving a porous structure.
Depending upon the size of foam needed is possible to alter the expansion percentage of
the structure, factors affecting expansion rate include;
• Time to cool
• Temperature it is pressed at
• Temperature of foam
• Percentage of foaming agent
• Percentage of additives added
Typically pure aluminium is used for this process due to its low melting point along with
good foaming qualities, yet using the right combination of blowing agent/matrix will more or
less allow any material can be used. The main problem with this process is that there is to
an extent randomness of the cells size, this increases the chances of voids to occur which
unfortunately makes it unreliable, especially when compared to some other methods.
To reduce the random cell growths it is possible to insert the pellet into a hollow mould,
then heat to expand. To increase flow conditions throughout the mould it is a good idea to
alter the precursors shape to manipulate its growth directions. Alternatively it can be
injection moulded during the expansion process allowing complicated shapes to be
created. This process does have the potential to be mass produced using the injection
moulding though testing would have to be extensive to find correct pellet shapes and

P a g e | 32
moulds custom designed to ensure maximum flow, with those reasons its costs will go up
dramatically and end up not being an economical method.
Aguirre-Perales, (2011) examined what factors affect a cellular materials expansion rates
using the powder compact melting technique. The foams that were created with a
combination of Aluminium (Al), tin (Sn) and TiH2 powder as a blowing agent. An Al-Sn
phase diagram can be seen in Figure 13, they mentioned how they used FTlite database
to calculate this stating that a liquid eutectic phase occurs at 232oC. However a debate
can be made if this is accurate as appendix 2 shows a completely different Al-Sn phase
diagram. The only reason this could be is that Figure 13 shows Al/(Al+Sn).
Figure 13, AL-Sn phase diagram. (Aguirre-Perales, 2011)
When the AI-Sn foams were made there were three important factors which affected
expansion growth;
1. Temperature pressed at.
2. Temperature foamed at.
3. Weight percentage of Sn (additive).
Temperature pressed at; two temperatures were used for this, 200oC and 300oC. It was
found that higher densities were obtained using the 300oC, densities of 98-99% whereas
200oC found densities of 95-96%. AR, (2003) reported that densification over 94% is
required to produce a good AL foam as it retains a high level of hydrogen due to the open
pores getting sealed under compaction. Another finding was that expansion rates

P a g e | 33
increased, this can be seen in Figure 14. The reason for increased expansion is due to the
Sn as it’s near its melting point as shown on Figure 13. As the Sn melts it fills voids
between the aluminium which in turn reduces the amount of blowing agent escaping.
Temperature foamed at; it can be seen in Figure 14 that a significant difference occurred
with the increase of temperature it was foamed at. All of the foams heated at 700oC show
crack like pores. 725oC showed small rounded pores when pressed at 300oC whereas at
200oC they are still in there early stages of forming. All samples at 750oC expressed full
development with pore coarsening.
Weight percentage of Sn; by increasing Sn it was found that expansion would increase for
both 700oC and 725oC, however not at 750oC. It is thought that at 750oC the foam reaches
it maximum at a faster rate thus Sn is not needed in high quantities to increase expansion
rate.
So to conclude it was found that Sn increases the expansion process of the foam, and
from an observation view hot pressing at 300oC with 3-5% Sn and foaming at 725oC
produced the best foams. Overall this journal seems very reliable with extensive research
being concluded including samples being attempted twice to decrease the chance of error.
Figure 14, Resulting foams from being tested at various pressing temperatures, temperature
foamed at and % of Sn. (Aguirre-Perales, 2011)

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Spray Foaming
Spray foaming is a completely different type of process compared to others which is still
being further developed to increase its quality and consistency. Commonly referred as to
“Osprey process” it is used to produce Billets or sheets, assuming parameters have been
selected appropriately.
Using an environmental chamber as seen in Figure 15 a metallic melt can be stored in a
crucible where it is then atomized and sprayed upon a substrate, once the spray begins to
mount upon the substrate it begins to cool and solidify. After a long period of time the
desired shape will be achieved and can be treated to remove defects/excess material. The
substrate can also be set to revolve to allow for a better distribution of the melt in case the
spray of metal is not even. It is crucial to ensure solidification doesn’t occur in the
segmented nozzle which sprays the liquid metal, to prevent it a water cooled nozzle is
used fitted with induction heating. Due to the high melting point required to melt the
aluminium alloy it would seem a better idea to use a ceramic for the nozzle rather than
having to treat copper to raise its melting point. However Yuan, (unknown) suggests;
‘The use of a copper nozzle ensures that no ceramic inclusions are re-introduced when the
liquid is poured from the crucible, as would occur with the ceramic nozzles used in
conventional spray forming.’
With no additives added to the melt the object created will just consist of a solid metallic
structure, so to make it porous and hence a foam the following needs to be done; Adding a
second spray consisting of oxides or carbides, ensuring the spray has been angled
appropriately they should create a homogenous distribution upon the substrate.
The added carbides/oxides will then decompose upon contact with the molten melt upon
the substrate and release gas to create a porous object. To ensure a consistently high
quality porous material is created it is essential that the sprays remain at the same height
from the substrate throughout the process, additives are sprayed evenly and constantly to
try prevent voids within the structure.
According to J.Banhart, (2001) it is possible to obtain the following combination of
characteristics;
‘Low oxide content, fine grain size or a high content of metastable alloy phases. This
combination of properties usually cannot be obtained by conventional casting methods.’
Although Yuan, (unknown) states that these characteristics can be achieved easily by

P a g e | 35
powder metallurgy alloying which asks the question why not use powder metallurgy
alloying if it is an established process; it is thought that this process once fully developed
will be a lot more economical.
Figure 15, Environmental chamber. (Oxford, 1995)
One problem with this process is that due to the randomness of the spray there can tend
to be defects which are a result of either one or a combination of the following factors;
• Insufficient liquid supply leading to interstitial porosity
• Blowing agent not decomposing fast enough leaving gas entrapment
• Shrinkage cause by the cooling of the metallic melt when solidifying
• Gas from the blowing agent causing precipitation

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2.4.2. Solid State Processing
The alternative to using molten method for creating cellular solids is using metallic
powders. Throughout the process the powder remains solid, the key is to use a heat
treatment techniques such as sintering or other solid state operations. When using liquid
state processing it was noticeable that the majority of them resulted in containing an open
cell structure, the opposite can be seen for solid state processing, closed cell. Methods of
producing these cellular materials include;
• Sintering of mixed powders to decompose a blowing agent.
• Entrapping gas within a powder compact.
• Compression and heat treatment of hollow spheres.
• Foaming of metallic slurries.
• Using space holders which can be removed or left in to create a syntactic foam.
Gas Entrapment
To begin this process a desired shape should be created consisting of a hollow centre, it is
essential that the thickness is bigger than required due to pressing at a later stage.
Typically used for this process is titanium and is currently being exploited by global aircraft
manufacturers Boeing. Once it has been cooled and the mould created a vacuum pump is
used followed by argon then being inserted. The tube allowing the argon to be inserted is
then crimped to prevent any argon to escape. With the use of a hot isostatic press applied
to the mould the internal pressure is said to rise to approximately eight times the original.
Although this is not high enough to deform the titanium a number of pores become present
within the structure, with the use of a rolling step the pores are spread out more uniformly
and flatten voids. According to H. Nakajima, (2013);
‘As the voids flatten, void faces come into contact and diffusion bond, creating strings of
smaller gas-filled pores.’
To increase uniformity of the argon bubbles cross rolling can be introduced. Lastly the
mould is put in a furnace for approximately a day where it is left at high temperatures to
again raise the internal pressure which will cause it to expand thus reducing the properties
density, typically left is a sandwich type construction which is highly sought for when
looking for lightweight materials. As the materials expansion occurs during a solid state
phase it is cannot be classed as a foam but that of a solid state creep process.

P a g e | 37
Typical porosities consist of approximately 20-40% meaning it consists of a higher density
than other cellular manufacturing techniques mentioned, theoretical calculations however
express a density of 50% is capable.
The large advantage of this process which is unquestionably why Boeing have applied this
technique for creating titanium structures is due to its possibility of creating sandwich
panels. Usually when creating sandwich structures there is extra requirement of an
adhesive or bonding method to bind the two metal sheets to an internal foam, this is not
needed using gas entrapment which enables the adhesive to not be a weak point/concern.
As sandwich panels become more popular because of the many advantages they contain
whilst relatively few disadvantages it is likely this process will become more popular. The
advantages and disadvantages of sandwich panels are;
• Advantages: it has high stiffness and strength whilst being lightweight. Without
drastically increasing the weight the internal core can be designed to allow for
higher stiffness and strength. Also, sandwich panels can function as thermal
insulation, this is dependent on the chosen material. Another advantage is its ability
to dampen vibration and noises which as a result could prevent mechanical failure.
• Disadvantages: they tend to be harder to manufacture which as a result cost more
to produce and some solids would have better strengths at a heavier weight.

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Foaming of Slurries
Foaming of slurries is a very basic production method, to start a slurry is created consisting
of metal powder and some sort of foaming agent. This combination should be mixed
thoroughly to create the best possible distribution of particles. The slurry is then poured
inside a mould where it is heated so that the foaming agent decomposes thus creating a
porous structure. This is then left to cool and a foam created, cooling rates can be altered if
different properties are desired.
Although this method can be used for metals it is typically exploited by ceramics instead.
When using metals the common choice tends to be aluminium powder with orthophosphoric
acid using with aluminium hydroxide as the foaming agent. Table 2 shows the advantages
and disadvantages of this process.
Another use of this method involves dipping an open porous polymer foam into the slurry,
once coated it is removed and dried before heating to a temperature which will disintegrate
the polymer leaving an open cell rigid metallic foam.
Table 2, advantages and disadvantages of foaming of slurries.
Advantages Disadvantages
Process is very simple allowing it to
be cost effective.
It is very hard to get an even distribution of
foaming agent/stabilisers throughout the
slurry.
Can achieve relative densities of
7%.
It is prone to cracks and voids within the
internal structure causing poor strength.
Near impossible to get a constant quality level
on bulk produced amounts.
Reaction Sintering
This process simply involves combing powders and lets the reactions caused between
them occur thus creating a porous structure as expressed by J.Banhart, (2001);
‘Reaction sintering of metal powder mixtures such as Ti + Al, Fe + Al or Ti + Si is also
known to yield porous structures. This is due to different diffusion coefficients of the
components of a multi-component system in each other.’

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Space Holders
To summarise space holders (section 2.4.1) a porous material can be obtained through
the use of a metallic melt being poured over a space holder which is later leached. The
difference between this process and section 2.4.1 is instead of using a metallic melt a
metallic powder is used. The fine metal powder and space holder are then combined
together, a substantial level of mixing is required to ensure significant distribution of the
space holder. Space holders can be ceramic particles, sand pellets, hollow spheres, salts
or metals with a lower melting point. These space holders are removed by heat treatment
or leaching via water. Once the space holder and powder has been mixed it is compacted,
either room temperature or hot pressing depending upon the space holder, hot pressing
will allow for an improved binding of the particles. If the space holder requires leaching it is
essential for high space holder percentage to interconnect the particles. Both Michailidis,
(2011) and Anon, (2012) found variance occurs with the size of pores in the final structure.
If further densification is required the structure can be sintered, Michailidis, (2011) looked
into this with aluminium powder and a leachable carbohydrate filler for the space holder.
Figure 16 shows SEM images of the cell walls, Figure 16; (d) shows how the walls were
before sintering, a large number of micro pores existed in the structure, one micro pore
can been seen in the image. However Figure 16; (a), (b) and (c) were sintered at 600 °C,
680 °C and 750 °C. It was found that by increasing the sintering temperature the micro
pores reduced and cell walls became denser which will result in an improved final product.
Figure 16; (c) displays how the aluminium has melted and flowed round the network of the
cell wall thus filling the voids.
Figure 16, SEM images of cell walls after sintering, (a) 600 °C, (b) 680 °C and (c) 750 °C, (d)
before sintering. (Michailidis, 2011)

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Hollow Spheres
The hollow spheres technique can be used to create open or closed cell structures in
ordered or disordered patterns, this is bone by the bonding of metallic spheres via
sintering and compression. Several methods exist for creating the hollow spheres;
• Chemical and electrical deposition of the metal on polymer spheres which can later
be removed with heat treatment.
• Coating polymer spheres with a powder consisting of metal and a binder which
again can be removed via heat treatment.
• Atomise a metallic melt and vary the parameters to allow the construction of hollow
spheres.
Once hollow spheres have been created they can either be randomly placed or aligned in
a mould depending on the characteristics wanted. The spheres are then sintered and
compacted to form a cellular material, the resulting structure before sintering can be seen
in Figure 17.
Figure 17, Before and after compression of hollow balls. (J.Banhart, 2001)
To increase bonding of the spheres a bonding slurry can be put used, this can also be
used as a coating to improve thermal resistance depending upon the slurry material. When
sintering it is good idea to compress it first, this will deform the spheres into polyhedral
bodies which will increase the level of sintering contacts although it will reduce the degree
of open porosity. By applying forces during sintering the spheres are deformed to
polyhedral shapes resulting in an increase of sintering contacts but also a reduction in the
degrees of open porosity. Koo, (2008) mentions that residual porosity in cell walls of the
material is governed by grain size of powder used to form spheres and sintering time.

P a g e | 41
2.4.3. Deposition Processing
Electro-Deposition
Electro-Deposition is a heavily material limited process, the only materials that can be
used are nickel or copper alloys. As with other section 2.4.3 methods a polymer precursor
is used to be built upon, it is possible for the precursor to contain any core shape such as
honeycomb etc. Electro-Deposition relies upon depositing metals onto an electrically
conductive material, as plastic is not electrically conductive the precursor needs a coating
which will allow the process to work. The precursor is coated by either cathode sputtering
or immersing it in a slurry based on graphite or carbon black. Once coated the copper or
nickel is deposited onto the structure and heat treatment is used to remove the precursor.
The end result will be an open cell foam containing hollow struts.
There are several modifications that can be done to the process;
By depositing alternating layers of nickel and chromium it is possible to then when heat
treating to cause a thermally induced interdiffusion which will result in a nickel-chromium
foam. This method is mentioned by J.Banhart, (2001), it seems like a very plausible
option.
Antenucci, (2014) investigated the improvements of mechanical and thermal
characteristics of coating an aluminium foam with copper using electro-deposition,
although this is not a manufacturing process it would replicate the effect of the hollow
struts within the foam being filled. Figure 18 shows an SEM of an aluminium foam
covered in copper. From coating the aluminium with copper there were several increased
properties; strength (MPa), strain, densification strength (MPa) and densification strain.
With the enhanced properties that are available it would be a good idea to fill the struts. If
the struts were filled constant heat is required when pouring to prevent solidification.
Figure 18, SEM of Aluminium foam coated in copper. (Antenucci, 2014)

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Vapour Deposition
Similar to lost wax method (section 2.4.1) the process involves a plastic precursor is used
as a template to build upon. With the use of a vacuum chamber metal typically nickel due
to its lower melting point is left to condense over the precursor which has been cooled to
increase the rate at which it does so. As the metal condenses over the precursor coats
precursor where it can be left to cool and then then sintered to remove the precursor. On
completion of sintering the final shape will consist of that of the precursor except
containing hollow centres. This method is also similar to that of electro-less deposition
which will be talked about in section 2.4.3.
As the metal condenses it forms layer around the precursor which build upon each other to
completely cover it. These layers can be seen in Figure 19 which although used nickel as
a precursor and coated with graphene the same principles applies.
Figure 19, SEM image of nickel coated graphene. (Trinsoutrot, 2014)
This method is typically exploited nickel carbonyl due to its low melting point however
copper and nickel can also be used. Controlling methods for the thickness of the metal
which will be deposited upon the precursor includes the length of time left to condense and
density of the metal chosen.
Trinsoutrot, (2014) Investigated using nickel as the precursor and graphene as the
material to condense found that two distinct disadvantages can be observed with the use
of vapour deposition disregarding material choice; there were numerous cracks on the
structure and high surface roughness.

P a g e | 43
Electro-Less Deposition
This process again is very similar to that of electro deposition (section 2.4.3) and vapour
deposition (section 2.4.3). Electro-less deposition for creating foams can be achieved by
two ways, either coating a foam or coating particles and compaction, both processes then
get sintered to remove the internal substrate. The coating particles and compaction
method is talked about in “Fabrication method of metallic closed cellular material”, section
2.4.3.
Typically used for electro-less deposition is nickel phosphorous however other noble
metals can be used such as gold or silver. For the rest of this process nickel will be
referred to as the material of choice, the process involves plating a plastic foam with a
nickel solution, the process is defined as an auto-catalytic chemical technique.
When plating the foam the solution must contain a nickel slat and hypophosphite as a
reducing agent, other chemicals are added such as ammonia to control the pH level and
also stabilising agents. As a result hydrogen ions and gases are by-products of the plating
procedure. After treatment to the foams surface it acts as a catalyst, the nickel deposits
also acts as a catalyst so the reaction continues auto-catalytically. Once the foam has had
a sufficient coating by leaving in the solution it can be sintered which results in the foam
being removed leaving a hollow strut nickel based foam. Unfortunately this process can
only create open cell foams, this is due to it replicating the structure of the initial foam. To
increase deposition rate heat should be applied to the nickel solution as it increases the
chemical reaction. Another factor to consider depending upon the application is the
percentage of phosphorus in the nickel solution as it will affect the mechanical properties.
Fan & Fang, (2008) investigated the difference between hollow-strut and solid-strut foams,
surprisingly they found some superior mechanical properties. They stated the these
properties were superior due to the following reasons;
‘Due to the enlarged bending stiffness of the hollow strut, the enhancements of stiffness,
buckling strength, plastic collapse strength, and brittle failure strength and fracture
toughness were substantial (even an order of magnitude) according to the analysis and
comparisons. The hollow-strut foam is much more damage tolerant than the solid-strut
foam.’

P a g e | 44
Fabrication Method of Metallic Closed Cellular Material
This manufacturing technique was conducted by S.Kishimoto and N.Shinya, they
published two journals on separate occasions about the process; S.Kishimoto, N.Shinya,
(2000) and S.Kishimoto, N.Shinya, (2001). The process involved coating a thermoplastic
polymer of 10-µm diameter with a 0.46-µm thick nickel–phosphorus layer which was
applied by an external company, as seen in Figure 20(b). Once coated the particles were
compressed using isostatic pressing at a force of 200 MPa, the press was also heated to
90°C to increase bonding, seen in Figure 20(c). Once compacted they were then sintered
for 1 h at 800°C in a vacuum atmosphere, the final result can be seen in Figure 20(d).
Figure 20, Schematic/SEM images of manufacturing technique. (S.Kishimoto, N.Shinya, 2001)
The two journals however show the same SEM images, this makes it seem as though the
process has not actually been replicated and that there is a chance the experiment does
not actually work. Another issue with the journals is that there is no photo showing the
foam created, only SEM’s.
Their conclusions found that it was possible to create a lightweight, high-energy absorption
and large ultrasonic attenuation coefficient cellular material. This meant it could be utilised
for applications that demanded noise prevention, energy absorption and passive-damping.

P a g e | 45
2.4.4. 3D Printing
3D printing is a conceptual idea for the production of cellular materials, over the last few
year huge advances have occurred with the process and it is unknown how far it can go in
the future.
Hewitt, (2012) mentions that NASA have come up with a technique call selective metal
melting (SLM), the process allows complex geometry’s to be created via the use of an
AutoCAD model at extreme accuracy. The process uses metal powders and binders which
are applied in layers, every layer is sintered via a moving laser which melts the binder and
temporary holds the structure together. Once the process is finished it can be removed
from the excess powder and heated to fuse together the metal. In theory this method does
seem plausible and cellular materials could be obtained however the literature should not
be completely trusted. An example of how the process works can be seen in Figure 21.
Figure 21, SLM method. (Hewitt, 2012)
PwC, (unknown) expresses that at the present steel, aluminium, and titanium are current
material options, the drawback being that they require laser melting machines which cost
in the range of $500,000 to millions of dollars each. These 3D printed objects are also
metallurgic ally different from machined part, it introduces voids and different metallurgical
grain structures. This means the structural integrity of these parts will not be sufficient for
most applications.
From a long term point of view the possibilities of 3D printing would allow manufacturing of
cellular materials to be created on demand at low costs, yet also being environmentally
due to little scrap material produced. The one large disadvantage is due to bonding being
necessary it is unlikely for the production of any flexible materials.

P a g e | 46
2.5. Applications of Cellular Materials
Foams are currently exploited for numerous applications due to their wide array of
properties which are ever expanding due to new techniques and materials being
developed. There are six main areas in which cellular materials are heavily used, an
additional concept idea has been mentioned has seen increasing interest is heat sinks,
this is mentioned in section 6.7.
2.5.1. Filters
Filters are one of the most common applications for cellular materials, although they tend
to be manufactured from plastic there are metallic ones. Plastics are typically used for
filters due to lower melting temperatures and cheaper material costs resulting in a more
economical filter. When increased temperatures or denser fluid requirements are required
metals are exploited. A filter is determined by its filtration capacity, particle retention,
corrosion resistance and arguably the most important, cost. One reason for cost being the
biggest decider upon selecting a filter is that filters do not actually remove debris or
bacteria from the flow, instead they trap it which eventually cause clogging. If this filter is
used extensively then replacements will be often which will eventually result in high
running costs.
2.5.2. Heat Exchangers and Cooling Machines
Open cell highly conductive metal foams are ideal for heat exchangers, heat can be
removed or added to gasses or liquids by letting them flow through the foam, at this point
the foam is heated or cooled. The problem is compromise has to be met when selecting a
foam, the two ideals are high thermal conductivity with low gas or liquid flow resistance,
and these however are contradicting ideals. As Haack, et al, (unknown) mentions the
reason for contradicting ideals are that smaller pores achieve higher rates of heat transfer
due to the larger area surface. Another important consideration that needs to be looked
into with heat exchangers is the bonding between the foam and solid material at which the
heat is transferred.
An alternate concept that does not be seen to be in existence would be to design a foam
with large hollow struts, these hollow struts would then allow pressurised fluid to be flown
through which in turn would increase cooling or heating. The struts would require

P a g e | 47
additional treatments to prevent cracks however as a concept there is no reason why it
wouldn’t work, especially when it is possible to fill the struts will a molten material as some
processes do.
2.5.3. Noise Dampeners
With residential areas constantly increasing further development is required on reducing
noise pollution, although this is not the reason for all noise reduction it is one of the most
common. When referring to noise reduction any moving components can be considered,
Woods, (09) states that NASA researched on metallic foams for reducing noise pollution
as planes caused disturbance to people living around airports. NASA found that by
optimising the metallic foams pore sizes and density it was possible to create an optimal
foam for the noise frequencies given out by the engines. After a large amount of testing
there was a reduction of 4-5 decibels up to more than fifty percent depending upon the
operating range of the engine. This shows how affective metallic foams can be as noise
dampers whilst providing extra strength in the structure.
2.5.4. Supports for Catalysts
Catalysts are used for accelerating chemical reactions whilst having the potential to drive
them along pathways through possible reaction networks, as a result the efficiency is
increased by avoiding undesirable by-products. To further enhance the effectiveness a
highly porous metallic foam can be produced to increase the surface area thus increasing
the area between the gases or liquid with the catalyst, as a result the reaction will occur
faster.
2.5.5. Flame Arresters
A flame arrester is a device that is fitted to the enclosure of a pipe containing flammable
vapours or gases, under normal circumstances the gases or vapours can flow easily
however if an ignition was to occur flames would be blocked from escaping. This device
can prevent catastrophic damage to equipment, loss of product and potentially save a life.
Limited, (unknown) explains that a flame arrester typically comprises of an element and a
housing/connections to create a tight seal around the pipeline. The foam which is often

P a g e | 48
referred to as an element consists of a porosity, these pores will then break down the
flames into smaller ones which are cooled as a result of the high heat capacity from the
element thus extinguishing the flames. It should also be noted that the element is designed
to create a pressure drop to obstruct the flames from gaining velocity. An example of this
flame arrester can be seen in Figure 22.
Figure 22, Flame arrester. (Limited, unknown)
2.5.6. Impact Energy Absorbers
As foams excel in energy absorption they can be exploited in any real application requiring
this property. One field with extensive research taking place is its use in cars, if the foam
was placed in structure of the vehicle it could save lives by absorbing energy in a crash
which in turn will increase the time at which the vehicle comes to a halt. Although these
foams have high energy absorption it typically results in the structure becoming fully
deformed, this means it should be used in safety applications rather than that of
applications relying upon the property frequently.
2.5.7. Heat Sinks
This conceptual idea has seen significant interest over the last few years, reasons being a
heat sink is used to increase air flow and in theory the pores of a foam would allow for
additional cooling. The only concern with this is that the pores would obstruct flow and as a
result cause turbulence, as a result it could end up being less efficient.

P a g e | 49
3. METHODOLOGY
The aim of this research project was to replicate several methods from production
methods identified and test the cellular materials. After identifying a large number of
recognised and novel production methods there were a lot of problems identified with
health & safety and university facilities. Health & safety was the biggest factor faced with
manufacturing cellular materials, potential safety factors involved the risk of death, a more
in-depth analysis can be read in section 3.1.1. The other restriction was with university
resources, the main reason being due to specialised equipment being required, refer to
section 3.1.2 for more information. Once the processes had been replicated the cellular
materials would undergo various tests, from a microscopic to resilience testing. It should
be noted that the resilience test rig was created for a student’s project thus accuracy is not
expected to be one hundred percent.
3.1. Limitations
3.1.1. Health and Safety
The first problem faced was storing TiH2, Alibaba, (unknown) supplies TiH2 particles and
mentions that it has to be stored in a shady, cool and ventilated area. Also it must be kept
in a vacuum environment to prevent oxidisation of the powder. Although this criteria could
be met it is very problematic due to the number of requirements needed, primarily the
vacuum one.
The next problem with using TiH2 is according to Sigma-aldrich, (unknown) the following
safety precautions are required; Eye shields, full-face particle respirator and Gloves. This
made it apparent there are serious risks involved with the blowing agent and thus it was
avoided.
Another issue with blowing agents was faced, as the material/blowing agent melting point
is significantly high and small explosions occur due to the blowing agent it would not be
ideal for molten material to splutter. If it did go badly there would be a high chance third
degree burns would occur.
Molten metal leads onto another safety issue, for instance with casting around space-
holders (section 2.4.1) molten metal would be poured into a small mould. As a result there
is always the chance it could land on someone, this would then require an external

P a g e | 50
technician to complete the process. After the ordeal of pouring the chances are the metal
would solidify to fast, as a result it would not work and more technological steps would
need to be taken to prevent it occurring.
As an easy to remove space holder would be desired there is only really one option,
sodium chloride. Unfortunately if it melted due to the heat of the melt chlorine gas would
be released. Chlorine gas was originally experimented throughout both world wars as a
chemical weapon to kill people, with this in mind it is not a sensible idea to endanger
people’s lives.
Because of these health and safety issues a lot of processes had to be disregarded.
3.1.2. University Resources
After removing processes due to the health & safety factors there were still some methods
possible. The next stage was to identify what resources would be required to replicate the
process and compare those vs the university resources. The limitations faced were;
Although a piston was available for pressing there was no perforated piston for replicating
the melt-squeezing procedure (section 2.4.1), as this was crucial to the procedure the
process could not be carried out.
As direct foaming is a specialised production method there is no direct foaming equipment
owned by the university, as a result all kinds of direct foaming methods are eliminated from
replication.
Although the university has several 3D printers there are no metallic ones, this is due to
the fact they are still in early stages of designing so costs are too high for the university.
There are currently several ABS and PLA 3D printers.
Unfortunately there are no hot press machines or isostatic presses, this does not prevent
any methods to be conducted. This does however limit the potential of enhancing the
properties obtained by the cellular materials.
Finally a blowing agent that could be used was strontium carbonate (SrCO3), the amount
needed would only be about 1% weight of powder used. However its melting point is
approximately 1494oC which the furnace is not able to operate to.

P a g e | 51
3.2. Procedure
After eliminating processes due to health & safety and University limitations two
experiments were possible, one metal and one plastic; Fabrication method of metallic
closed cellular material (section 2.4.3) and 3D printing (section 2.4.4). As previously
mentioned there are doubts about how the resulting cellular material will be using the
“fabrication method of metallic closed cellular material method” due to the published
journals. To check there results the procedure was replicated as closely as possible with
the limitations faced. As there was very limited literature on 3D printing cellular materials
no actual procedure could be replicated, this meant the procedure was improvised.
3.2.1. Fabrication Method of Metallic Closed Cellular Material
A thermoplastic polymer, polystyrene particles were ground down to fit through a sieve of
200µm, however as the grinder could not grind particles to that dimension the sieve was
increased to 460µm; 8grams of polystyrene particles were achieved. The beige particles
then underwent several processes before coating with the electro-less nickel solution;
1. Particles were stirred in cuprolite X96 at 30oC for 5 minutes, this was to clean the
particles.
2. Particles were stirred in a pre-catalyst at room temperature for 2 minutes, this was
to prepare them for the catalyst.
3. Particles were stirred in a catalyst at approximately 38oC for 5 minutes, it was then
rinsed with de-ionised water. This altered the colour of the particles of beige to
black, this was not expected.
4. Placing the particles in a dry oven at 80oC overnight to remove any excess liquid.
Each process required the use of a Buchner funnel to remove the liquid from the particles,
filter paper was also placed inside the funnel to cover the perforations in the funnel thus
preventing loss of the particles. De-ionised water was sprayed on the paper to prevent any
air pockets occurring and securing it.
At this stage it became clear there was enough particles to provide more than one sample,
so the material was split into approximately thirds allowing for three samples. The three
samples would allow for alternate nickel thicknesses, however as nickel thickness would
only be established by checking it under a microscope, three coating timescales were
chosen; 40 minutes, 1 hour and 1 hour 40minutes.

P a g e | 52
The electro-less nickel was produced by combining 0.3L of SLOTONIP 1851 starter, 0.12L
of SLOTONIP 1853 replenisher, deionised water to the working level and finally 50%
ammonia to increase the pH level to 4.9. By increasing the pH level the phosphorus level
is decreased which in turn will alter the microstructure of the final sample. The final nickel
solution was considered medium phosphorus, 6-9% by weight.
The particles were coated for their chosen timescale in a temperature region of 85-91oC
with 89oC being the ideal; highest nickel deposition rate. During the coating the stirrer must
retain a high RPM, if not the particles remain at the top of the fluid due to their low density
resulting in a limited coat.
After coating the particles in nickel they were placed in a dry oven at 80oC overnight, this
allowed for a 25 tonne ring press to apply an axial pressing at 10 tonnes to compress the
particles into pellets. These particles were then sintered at 800oC for 1 hour at 20oC per
minute.
3.2.2. 3D Printing
As mentioned in (section 3.1.2) there is no metal 3D printer at the university, so a PLA
version was used, it is accurate to 0.2mm. Even though the aim was to replicate metallic
cellular materials it would be a worthwhile to see if the process was even plausible with the
use of polylactic PLA.
An AutoCAD model was created of a cellular material, this can be observed in Figure 23,
as the pores are internal the best way to emphasise the internal design was to display the
image by hidden edges. By creating a cube of 1x1x1 cm and cutting out an internal
sphere it was possible to replicate it by 5 times every direction. This allowed a 5x5x5 cm
box to be created which would be defined as a cellular material due to the spherical pores.
There was the option of using acrylonitrile butadiene styrene (ABS) as the material for the
3D printer however PLA was chosen because of its ability to cool and set extremely fast
whilst producing no harmful vapours during melting.

P a g e | 53
Figure 23, AutoCAD model for 3D printing.

P a g e | 54
4. Results
4.1. Fabrication Method of Metallic Closed Cellular Material
After compaction via the 25 tonne ring press at 10 tonnes the pellets seen in Figure 24
were created. For future reference (1) is the 40minute coating, (2) is the 1 hour coating
and (3) is the 1 hour 40 minute coating.
Figure 24, Compacted pellets: (1); 40 minutes, (2); 1hour
After sintering pellet (1) & (2) for 1 hour at 800oC there was a noticeable change, this can
be observed in Figure 25, an additional image can be seen in Appendix 3.
Figure 25, Sintered pellets: (1); 40 minutes, (2); 1hour.
Pellet (3) after being sintered can be seen in Figure 27, Figure 26 was taken through the
use of a microscope to identify a better understanding of the pores.
Figure 26, Pellet (3) microscope image.Figure 27, Pellet (3) after sintering.
1 2
21
3

P a g e | 55
To further identify the structure of sintered pellet (3) a scanning electron microscope
(SEM) was used. Several images were taken to gather a better understanding, Figure 28
shows the top of the structure whereas Figure 29 identify pores on the structures side.
Figure 29, SEM showing exposed pores on the side of sintered pellet (3).
Figure 28, SEM displaying the top of sintered pellet (3).

P a g e | 56
To gain further understanding an elemental analyses was conducted to identify the weight
percentages of elements within sintered pellet (3). To ensure no misreading’s or variations
occurred across the cellular material four elemental analysis’s were taken. Figure 30/31
shows two of the reports taken, a further two can be observed in Appendix 4/5.
Figure 30, Report 1 of elemental analysis.
Element Weight percentage (%)
Oxygen 0.82
Phosphorus 0.26
Nickel 98.92

P a g e | 57
`
Figure 31, Report 2 of elemental analysis.
Element Weight percentage (%)
Oxygen 3.07
Phosphorus 6.66
Nickel 90.27

P a g e | 58
4.2. 3D Printing
After the 3D printer ran the AutoCAD file the resulting PLA cellular material looked like
Figure 32.
Figure 32, 3D printed cellular material.
To conduct whether the resilience test was possible a displacement analysis was created
on AutoCAD, this was to ensure the cellular material could withstand the test at 7.5kg of
weight added. Results are displayed in Figure 33.
Figure 33, Displacement analysis.

P a g e | 59
Care was taken when using the resilience test, to ensure the correct data was taken the
test was replicated three times with average values used. The Acceleration with G-Force
results are displayed in Figure 34.
Figure 34, Resilience test. (Ellam, 2014)
0.001
0.023
0.045
0.067
0.089
0.111
0.133
0.155
0.177
0.199
0.221
0.243
0.265
0.287
0.309
0.331
0.353
0.375
0.397
0.419
0.441
0.463
0.485
0.507
0.529
0.551
0.573
0.595
0.617
0.639
0.661
0.683
0.705
0.727
0.749
0
5
10
15
20
25
30
35
40
45
Time(S)
Acceleration(m/s2)
Acceleration with G-Force

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