This document is a report submitted by Mohit Rajput in partial fulfillment of the 9th semester evaluation of an Integrated Dual Degree program in Metallurgical and Materials Engineering at IIT Roorkee. It discusses controlling the structure of metal foam using microwave heating. Metal foams can be produced by various methods, with powder metallurgy being able to produce closed cell foams in near net shapes. This study will use powder metallurgy as the basis and apply microwave heating to better control the foam structure, as microwave heating provides advantages like faster and more uniform heating from within compared to conventional outside-in heating. The effects of foaming temperature, time and other parameters on the foam structure will be investigated

1. 1
Report on
CONTROL OF METAL FOAM STRUCTURE USING
MICROWAVE HEATING
Submitted in partial fulfilment of 9th
semester evaluation of
Integrated Dual Degree
by
Mohit Rajput (12216014)
5th
year, IDD student
Under the Supervision of
Dr. B.S.S. Daniel
Professor
DEPARTMENT OF METALLURGICAL AND MATERIALS
ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY ROORKEE
ROORKEE-247667
November, 2016

2. 2
Candidate Declaration
This to certify that the work which is being in this dissertation entitled “Control of Metal
Foam Structure using Microwave Heating” the partial fulfilment of 9th
semester
evaluation of Integrated Dual Degree (Bachelor of Technology + Master of Technology) n
Metallurgical and Materials Engineering, IIT Roorkee is an authentic record of my own
work done during June 2016 to November 2016 under the guidance of Dr. B.S.S. Daniel,
Professor, Department of Metallurgical and Materials Engineering, IIT Roorkee.
Mohit Rajput
Date: 21 November 2016
Place: IIT Roorkee
Certificate
This is to certify that above declaration made by the candidate is correct to the best of my
knowledge.
Dr. B.S.S. Daniel
Professor
Department of Metallurgical and Materials Engineering
IIT Roorkee

3. 3
Table of Content
Heading Page
No.
Abstract
Chapter 1: Introduction 1
Chapter 2: Literature Review 3
2.1 Introduction to Cellular Materials 3
2.2 Metal Foams Advantages and Applications 5
2.3 Production Route for metal Foam 6
2.4 Production Techniques 8
2.4.1 Foaming of Melts by Gas Injection (ALCAN / HYDRO) 8
2.4.2 Foaming of Melts with Blowing Agents (ALPORAS) 9
2.4.3 Solid-Gas Eutectic Solidification (GASAR) 10
2.4.4 Foaming of Powder Compacts (FOAMINAL / ALULIGHT) 11
2.4.5 Foaming of Ingots Containing Blowing Agents(FORMGRIP / FOAMCAST) 12
2.4 Heat Treatment 13
2.5.1 Microwave based material processing method 13
2.5.1.1 Heating mechanisms in microwave materials processing 18
2.5.1.1.1 Heating mechanisms in non-magnetic materials 18
2.5.1.1.2 Heating mechanisms in magnetic materials 19
2.5.1.2 Microwave Heating techniques and heat transfer modes 19
2.5.1.2.1 Direct heating 19
2.5.1.2.2 Selective heating 20
2.5.1.2.3. Hybrid heating 20
2.5.2 Convention and Microwave based Material Processing 22
Chapter 3: Plan of Work / Methodology 25
Chapter 4: Experimental Procedure 26
4.1 Introduction 26
4.2 Materials used 26
4.3 Procedure 27
4.3.1 Green Compact Preparation 27
4.3.2 Heat Treatment 28
4.4 Microstructure 37
4.4.2 Sample Preparation 37
4.4.2 Microscopy 37
Chapter 5: Results and Discussion 38
Chapter 6: Summary 42
References 44

4. 4
Table of Figure
Heading Page
No.
Fig 1 - Dispersion of two phases. Each phase could be one of the three states of matter. [11] 3
Fig 2 - Metal Foams example 4
Fig 3 - Structural element of metal foams (a) Closed-cell & (b) Open-Cell [12] 5
Fig 4 - Various families of production methods for cellular materials [10] 7
Fig 5 - A family tree of metal foams [28] 7
Fig 6 - The range of cell size and relative density for the different metal foam manufacturing
methods
8
Fig 7 - Direct foaming of melt (ALCAN / HYDRO) 9
Fig 8 - Direct foaming of melt (ALPORAS process) 10
Fig 9 - Foaming metals by GASAR process 11
Fig 10 - Powder metallurgical process for foamed metals 12
Fig 11 - Metal foaming of ingots 13
Fig 12 - Favourable characteristics of microwave materials processing [54] 14
Fig 13 - Development of microwave processing of materials and their application areas (T –
processing temperature).
15
Fig 14 - Microwave energy absorption as a function of electrical conductivity 16
Fig 15 - Microwave interaction with materials. (x-axis represent dielectric loss factor) [61] 16
Fig 16 - Heating mechanism in dipolar loss. 18
Fig 17 - Heating mechanism in conduction loss. 19
Fig 18 - Types of microwave heating (a) direct heating, (b) selective heating and (c) hybrid
heating
20
Fig 19 - Bird's eye view of microwave materials processing 21
Fig 20 - Heating mechanism for (a) conventional Heating from outer to inner surface and
(b) microwave heating from inner to outer surface
22
Fig 21 - Comparison of heating procedure b/w microwave and conventional method [65] 23
Fig 22 - Temperature distribution in conventional, microwave, and microwave hybrid heating[54] 24
Process Methodology / Plan of Work 25
Fig 23 - Effect of Uniaxial Pressure on Green Density with powder size between 2-150 µm [88] 27
Fig 24 - Final Sample Images after compaction 28
Fig 25 – Microstructure of final sample B2-1 37
Fig 26 – Leica Inverted Microscope 37
Samples Final Images 38

5. 5
Table of Tables
Heading Page
No.
Table 1 - Potential applications for metal foams 5
Table 2 - Acquired Material Property Table 26
Table 3 - Equipment used for heat treatment 29
Table 4 - Experimental setup, heat treatment and final result for each sample 30
Table 5 - Sample Treatment Description 39

6. 6
ABSTRACT
Metal foam can be produce by various method and much work has been done in improving
these production methods. Production method for metal foams can be divided into casting,
metallic deposition, powder metallurgy and sputter deposition. Each method results in its
own characteristic density range, cell shape and sizes.
This current study will try to develop a method through which metal foam structure can be
controlled more precisely. Powder metallurgy route as the basis for the production of metal
foam has been adopted because of it is among the niche process which is able to produce
close cell foams with additionally producing a near net shape and complex foam part. For
further controlling the metallic foam structure microwave heating method will be studied
and results will be compared with the samples produced by conventional heating method.
Both conventional and microwave heating method has their ups and down while
conventional heating can be used for any material microwave heating is dependent on
microwave-material interaction which is also a function of time. In general microwave
heating is very rapid also heat is generated from within the material then the conventional
outside-in heating. It is observed that with the inside-out more homogenous foaming took
place compared to the conventional outside-in heating. Metals at room temperature are
opaque to microwave using a susceptor to increase the rate of process is adopted also pairing
the powder approach with microwave heating is beneficial as metal powder is reported to be
good absorber and gets heated effectively compared to bulk metal. Porosity of the compact
plays an important role in the foaming as more porous pellet wouldn’t be able to trap the gas
after the foaming agent decomposes while a lesser porous pellet will have a lesser heating
rate with microwave interaction. Foam structure could be further controlled by oxidizing the
surface of foaming agent or coating the particles. This study will also deal with finding the
balance amount of silicon carbide with in the matrix to control the viscous flow while
foaming also for the purpose of susceptor with microwaves. Result with foaming
temperature and foaming time will be studied.

7. 1
Chapter 1
Introduction
Cellular material unique properties have gathered the interest of researchers for scientific
and industrial application. Metal foams have promising application in various field namely
aerospace, structural, automotive, biomedical, Insulation and even for ornamental purposes
because of it material and structure making it preferred choice cause of its properties such
as high specific strength, high energy absorption of either heat or impact or vibration or
sound with good efficiency, high stiffness, and even osteoconductive property with
magnesium foam. [1-5]. Foam properties like mechanical, electrical, thermal and damping
are a related cellular structure and the properties of material present. The main structural
parameters are relative density, the degree to which cell are open or closed, average cell
size, average cell all thickness, and the degree of anisotropy [7].
Porous metals and metallic foams possess advantageous properties when compared with
bulk ceramics, polymers and metals or polymer and ceramic foams. When mechanical
properties are concerned metallic foams strength, stiffness and energy absorption are much
higher than those of polymer foams also they are able to maintain their mechanical
properties at much higher temperatures than polymers. Besides, they are generally more
stable in harsh environments than polymer foams. Additionally, they are thermally and
electrically conductive and they have the ability to deform plastically and absorb energy,
as opposed to ceramics. If metal foam has open porosity through it liquids or gasses would
be able to pass through i.e. they will be permeable and with their high specific surface area
they can have excellent surface heat exchange. [6]
Metal foam are classified in two categories based on the cell interconnection i.e. if gaseous
phase is not interconnected in the foam it is referred to as closed cell foam else if the cells
are interconnected it is referred to as open cell foam. The Ideal foam has pores of fairly
uniform size distributed evenly throughout the structure [8]. The characteristic density
range, cell shape and sizes of metal foam will be function of production method used.

8. 2
Powder metallurgy route is among the niche process which is able to produce close cell
foams with additionally producing a near net shape and complex foam part [9]. Use of
Microwave heating has recognized various advantages namely large energy saving cause
of much smaller cycle time, fine microstructure, improved mechanical properties, eco-
friendliness, and selective and volumetric heating [54,61]. More refined properties can be
achieved using microwave heating it rather than with conventional heating. Both
conventional and microwave heating method has their ups and down while conventional
heating can be used for any material microwave heating is dependent on microwave-
material interaction which is also a function of time. In general microwave heating is very
rapid also heat is generated from within the material then the conventional outside-in
heating. It is observed that with the inside-out more homogenous foaming took place
compared to the conventional outside-in heating. Metals at room temperature are opaque to
microwave using a susceptor to increase the rate of process is adopted also pairing the
powder approach with microwave heating is beneficial as metal powder is reported to be
good absorber and gets heated effectively compared to bulk metal. Porosity of the compact
plays an important role in the foaming as more porous pellet wouldn’t be able to trap the
gas after the foaming agent decomposes while a lesser porous pellet will have a lesser
heating rate with microwave interaction. Foam structure could be further controlled by
oxidizing the surface of foaming agent or coating the particles. This study will also deal
with finding the balance amount of silicon carbide with in the matrix to control the viscous
flow while foaming also for the purpose of susceptor with microwaves. Result with
foaming temperature and foaming time will be studied. Additionally, the foaming result
with the microwave heating will be compared to that of the conventional heating.

9. 3
Chapter 2
Literature Review
2.1 Introduction to Cellular Materials
For understanding the term form firstly, we need to understand the term dispersion of
phases which can be understood using the fig. shown below. The original sense of the term
“foam” is reserved for dispersion of gas in liquid and by letting this dispersed phase to
solidify will yield a structure called as “solid foam”. When the material for the production
is metal this solid foam is termed as “metal foams” [10].
Fig 1. Dispersion of two phases. Each phase could be one of the three states of matter. [11]
Metallic foams and cellular metals the structure made up of metals and pores are
voluntarily embedded in the structure. There are various terms related to these structures
which are often confused these are namely foamed metal, metal foams, cellular metal,
porous metal and metal sponge. While foamed metal and metal foam are the same, porous
metal is a subset of cellular metals. While the cellular metals are generally referred to
metal having large volume of porosities, metal foams are related to porous metal produced

10. 4
from a foaming process i.e. where foaming took place and the structure with highly
porous, complex and interconnected porosities are referred to as metal sponges, in these
cells are not well defined. [8,28] Though strictly speaking the term metal sponges i.e.
interconnected cells should only be used but they are often termed as open-cell metal foam
while the separated cell foams are referred to as Closed-cell metal foam. See Fig 2 Hence
foam can be classified in two categories based on the cell interconnection i.e. if gaseous
phase is not interconnected in the foam it is referred to as closed cell foam else if the cells
are interconnected it is referred to as open cell foam. The Ideal foam has pores of fairly
uniform size distributed evenly throughout the structure [8]. The characteristic density
range, cell shape and sizes of metal foam will be function of production method used.
Close-cell Al foam Extended cellular iron
based foam
Al sponge / Open-Cell
foam
Nickel sponge / Open-
Cell foam
Fig. 2 – Metal Foams example
People have already known about cellular materials but may have not realised about it
these are present everywhere in our environment like in tree, leaf and even our bone are
the example of natural cellular structure. These structures have promising applications in
many field such as aerospace, structural, automotive, biomedical, Insulation to name a few.
Hence these materials have gathered the interest of researchers for scientific and industrial
application. Metals Foam is one of these structures which shows properties such as high
specific strength, high energy absorption of either heat or impact or vibration or sound
with good efficiency, high stiffness, and even osteoconductive property with magnesium
foam. [1-5]. Foam properties like mechanical, electrical, thermal and damping are a
related cellular structure and the properties of material present. For these materials, the
main structural parameters are relative density, the degree to which cell are open or closed,

11. 5
average cell size, average cell all thickness, and the degree of anisotropy [7]. shown in Fig
3.
Fig 3 – Structural element of metal foams (a) Closed-cell & (b) Open-Cell [12]
2.2 Metal Foams Advantages and Applications
Porous metals and metallic foams possess advantageous properties when compared with
bulk ceramics, polymers and metals or polymer and ceramic foams. When mechanical
properties are concerned metallic foams strength, stiffness and energy absorption are much
higher than those of polymer foams also they are able to maintain their mechanical
properties at much higher temperatures than polymers. Besides, they are generally more
stable in harsh environments than polymer foams. Additionally, they are thermally and
electrically conductive and they have the ability to deform plastically and absorb energy,
as opposed to ceramics. If metal foam has open porosity through it liquids or gasses would
be able to pass through i.e. they will be permeable and with their high specific surface area
they can have excellent surface heat exchange. Structure similar to shown in fig. 1b, i.e.
two-dimensional honeycomb has found many application in aviation industry. [1-6, 13-27]
Various other applications are shown below in table1.
Table 1: Potential applications for metal foams
Applications Comments
Light-weight
structures
Metal foams have good stiffness-to-weight ratio when loaded in
bending.
Sandwich cores Metal foams have low density with good shear and fracture strength.
Strain isolation Metal foams can take up strain mismatch by crushing at controlled
pressure.
Mechanical damping The damping capacity of metal foams is larger than that of solid metals
by up to a factor of 10
Vibration control Foamed panels have higher natural flexural vibration frequencies than
solid sheet of the same mass per unit area

12. 6
Acoustic absorption Reticulated metal foams have sound absorbing capacity.
Energy management:
compact or light
energy absorbers
Metal foams have exceptional ability to absorb energy at almost
constant pressure.
Packaging with high-
temperature
capability
Ability to absorb impact at constant load, coupled with thermal
stability above room temperature.
Artificial wood
(furniture, wall
panels)
Metal foams have some wood-like characteristics: light, stiff, and
ability to be joined with wood screws.
Thermal
management: heat
exchangers,
refrigerators
Open-cell foams have large accessible surface area and high cell-wall
conduction giving exceptional heat transfer ability
Thermal
management: flame
arresters
High thermal conductivity of cell edges together with high surface area
quenches combustion.
Thermal
management: heat
shields
Metal foams are non-flammable; oxidation of cell faces of closed-cell
aluminium foams appears to impart exceptional resistance to direct
flame.
Consumable cores
for castings
Metal foams, injection-moulded to complex shapes, are used as
consumable cores for aluminium castings
Biocompatible inserts The cellular texture of biocompatible metal foams such as titanium
stimulates cell growth
Filters Open cell foams with controlled pore size have potential for high-
temperature gas and fluid filtration.
Electrical screening Good electrical conduction, mechanical strength and low density make
metal foams attractive for screening
Electrodes and
catalyst carriers
High surface/volume ratio allows compact electrodes with high
reaction surface area.
Buoyancy Low density and good corrosion resistance suggests possible floatation
applications.
2.3 Production Route for Metal Foam
Metallic foam can be produce using ways which can be divided in the following four
classes:
• when foam is formed from the vapour state of metal
• when foam is formed from the liquid state of metal
• when foam is formed from the solid state of metal
• when foam is electrodeposited from an aqueous solution.
Below Fig 4 and 5 are shown to help in distinguishing the processes.

13. 7
Fig 4 - Various families of production methods for cellular materials [10]
Fig 5 - A family tree of metal foams [28]
Fig 5 shown above provides an overview of the methods available for making metal
foams. One distinguishing factor is whether molten metal or metal powder is used
(although the actual foaming always takes place in the liquid state). A second difference is
the gas source used for creating porosity: an external source can be used, a blowing agent
can be decomposed in-situ, or dissolved gas can be forced to precipitate. Third, foaming
can be instantaneous (i.e., addition of gas leads to immediate foaming), or an intermediate
product is created that can be foamed in a later stage (delayed foaming). Some methods
have been given a name, others were given a commercial name by the manufacturer.
Each of these processes results in the characteristic structure, size and regularity of the
cells and relative density of the foam. Majority of processes produces close-cell foam and
some produces open cell foam. Fig 6 shown below showcase some of these characteristics
based on the process used.

14. 8
Fig 6 - The range of cell size and relative density for the different metal foam manufacturing
methods
2.4 Production Techniques
2.4.1 Foaming of Melts by Gas Injection (ALCAN / HYDRO)
This technique is also known as direct foaming of melts. This technique is based on
injecting gases into the liquid melt for causing the melic melt to foam under some
maintained circumstances.
Since foam is a dispersion of gas and liquid gas because of buoyancy tends to rise up to the
surface of the liquid and since in metallic melt because of high density difference the gas
bubble will quickly rise up because of higher buoyancy force. To tackle this problem melt
is made viscous to slow down the gas rising velocity such that the melt will have enough
time to get solidified. The additive to make melt viscous are generally ceramic powder or
alloying elements. These additive generally include fine ceramic particle such as silicon
carbide, aluminium oxide or magnesium oxide.
The advantage of this process is the ability of producing large volumes at a rather low cost
and the low density that can be achieved. Porosities range from 80 to 97%. A possible
disadvantage is the eventual necessity for cutting the foam and therefore opening the cells

15. 9
and the brittleness of the MMC foam due to the reinforcing particles contained in the cell
walls.
The process has been depicted in the Fig 7, shown below.
Fig 7 - Direct foaming of melt (ALCAN / HYDRO)
Information about the practical implementation of this type of foams production can be
found from these literature [29-33] and some property of these form are given in the
literature [34-35].
2.4.2 Foaming of Melts with Blowing Agents (ALPORAS)
This technique for foaming melts uses the direct addition of foaming agent to the melt
instead of blowing gas into it [36,37]. In this process blowing agent decomposes under the
presence of high temperature causing the released gas to propel foaming process.
In this process calcium is mixed in aluminium melt at around 680 °C. The melt is stirred
for several minutes, during which its viscosity continuously increases by a factor of up to
five because of the formation of calcium oxide (CaO), calcium-aluminium oxide
(CaAl2O4), or perhaps even Al4Ca intermetallic, which thicken the liquid metal. After the
viscosity, has reached the desired value, titanium hydride (TiH2) is added (typically 1.6
wt.%), serving as a blowing agent by releasing hydrogen gas in the hot viscous liquid. The
melt soon starts to expand slowly and gradually fills the foaming vessel. The foaming

16. 10
takes place at constant pressure. After cooling the vessel below the melting point of the
alloy, the liquid foam turns into solid aluminium foam and can be taken out of the mould
for further processing.
The process has been depicted in the Fig 8, shown below.
Fig 8 - Direct foaming of melt (ALPORAS process)
One of the advantage of this technique is that pore structure of the material is rather
uniform and a possible disadvantage is that this is rather expensive than direct foaming.
Information about the practical implementation of this type of foams production can be
found from these literature [38] and some property of these form are given in the literature
[39,40].
2.4.3 Solid-Gas Eutectic Solidification (GASAR)
This technique exploits the fact that some liquid metals form a eutectic system with
hydrogen gas. If one of these metals is melted in a hydrogen atmosphere under high
pressure (up to 50 atms), the result is a homogeneous melt charged with hydrogen. If the
temperature is lowered, the melt will eventually undergo a eutectic transition to a
heterogeneous two-phase system (solid + gas). If the composition of the system is
sufficiently close to the eutectic concentration, a segregation reaction will occur at one
temperature. As the melt is solidified, gas pores precipitate and are entrapped in the metal.
The resulting pore morphologies are largely determined by the hydrogen content, the

17. 11
pressure over the melt, by the direction and rate of heat removal, and by the chemical
composition of the melt.
The process has been depicted in the Fig 9, shown below.
Fig 9 - Foaming metals by GASAR process
The possibility of solidifying the liquid directionally offers the advantage of making foams
with elongated pores. The pore structure of such foams is somewhat problematic [41,42],
so that further improvements have to be awaited.
Information about the practical implementation of this type of foams production can be
found from these literature [43] and some property of these form are given in the literature
[44,45].
2.4.4 Foaming of Powder Compacts (FOAMINAL / ALULIGHT)
The production process begins with the mixing of metal powders - elementary metals,
alloys or powder blends - with a foaming agent, after which the mix is compacted to yield
a dense, semi-finished product. In principle, the compaction can be done by any technique
that ensures that the foaming agent is embedded into the metal matrix without any residual
open porosity. Heat treatment at temperatures near the melting point of the matrix material
is the next step. During this process the foaming agent, which is homogeneously

18. 12
distributed within the dense metallic matrix, decomposes. The released gas forces the
compacted P/M material to expand thus forming its highly porous structure. The density of
metal foams can be controlled by adjusting the content of foaming agent and several other
foaming parameters such as temperatures and heating rates. If metal hydrides are used as
foaming agents, a content of less than 1% is sufficient in most cases.
The process has been depicted in the Fig 10, shown below.
Fig 10 - Powder metallurgical process for foamed metals
Advantage of this process is that quite complicated parts can be manufactured by injecting
the expanding foam into suitable moulds and allowing for final expansion there.
Information about the practical implementation of this type of foams production can be
found from these literature [46-48] and some property of these form are given in the
literature [49-52].
2.4.5 Foaming of Ingots Containing Blowing Agents (FORMGRIP/FOAMCAST)
The powder-compact melting process was modified by incorporating titanium-hydride
particles directly into aluminium melt instead of using powders to prepare a foam able
precursor material. To avoid premature hydrogen evolution the melt has to be either
quickly cooled down below its melting point after mixing or the blowing agent has to be
passivated to prevent it from releasing gas before solidification.
Achieving a homogeneous distribution of TiH2 powders in the die is challenging. The
latter route requires that TiH2 powders be subjected to a cycle of heat treatments that form
an oxide barrier on each particle and delay decomposition. The powders are then added to

19. 13
a melt and can be cooled at comparatively slow rates after stirring. Melts containing silicon
carbide are used to obtain stable foams. The foaming process can be influenced by varying
heating rates and final foaming temperatures, thus allowing for producing a variety of
different pore structures.
The process has been depicted in the Fig 11, shown below.
Fig 11 - Metal foaming of ingots
Further information can be found in these literature [52,53].
2.5 Heat Treatment
The produce compact will be given either the microwave heat treatment or conventional
heating for foaming to take place. Since microwave heating is the new material processing
method it is been discussed in detail.
2.5.1 Microwave based material processing method
Microwaves are electromagnetic waves which consist of an electric and a magnetic field
orthogonal to each other with wavelengths in the range of 1–1000 mm. Microwaves are
wave energy that is converted into heat energy depending upon the type of interaction with
the target materials. The processing of a material using microwaves depends on its
dielectric and magnetic properties as the electric field and magnetic field components
interact with the material during irradiation [61,62]and frequencies between 300 GHz to
300 MHz, respectively. However, only very few frequency bands in this range are allowed
for research and industrial applications to avoid interference with communication. The
most common microwave frequency used for research is 2.45 GHz (wavelength ~ 12.25

20. 14
cm), the same as for the domestic microwave ovens; the other allowable frequencies are
915 MHz (wavelength ~ 32.8 cm), 30 GHz (wavelength ~1 cm) and 83 GHz for some
specific applications [55].
Microwave technology is attractive because it has many obvious advantages when
compared with conventional methods, such as: very short cycle time resulting in energy
savings as high as 90% over conventional methods, rapid heating rates, finer
microstructures, and hence, improved mechanical properties and environmental
friendliness [57]. Microwave material processing technology has gained much interest due
to the relatively low manufacturing costs, both energy and time saving, the fast sintering
process, short soaking time, higher energy efficiency, improved product uniformity and
high yields [60]. Fig 12 below shows the variety of microwave material processing
characteristics.
Fig 12 – Favourable characteristics of microwave materials processing [54]
In microwave heating, the electromagnetic energy is absorbed by the material as a whole
(also known as volumetric heating) due to microwave-matter coupling and deep
penetration, and then is converted in to heat through dielectric (in case of ceramics),
magnetic permittivity/eddy currents (metals) loss mechanisms. Since there is an energy
conversion and no thermal conductivity mechanism involved, the heating is very rapid,
uniform and highly energy efficient. This processes are fundamentally different in heating
mechanisms, and hence often result in a vastly different product.

21. 15
Due to the internal heating in the microwave processing, it is possible to sinter many
materials at a much lower temperature and shorter time than required in conventional
methods. The use of microwave processing reduces typical sintering times by a factor of
10 or more in many cases, thereby minimizing grain growth. Thus, it is possible to produce
fine microstructure in microwave sintered metal parts [55].
The areas where it has been applied include: process control, drying of ceramic sanitary
wares, calcination, and decomposition of gaseous species by microwave plasma, powder
synthesis, and sintering of oxide ceramics and some non-oxide systems [56-59]. Till now
microwave has been utilized in variety of applications as illustrated below in Fig 13. These
applications involve different range of temperatures for processing and can be categorized
in low, moderate and high temperature processing groups as shown below.
Fig 13 - Development of microwave processing of materials and their application areas (T –
processing temperature).

22. 16
It was conventionally believed that all metals reflect microwave and/or cause plasma
formation and hence cannot be heated. However, this is only valid for sintered or bulk
materials at room temperature and not for powdered materials and/or at higher
temperature. This observation can be seen from the plot shown below in Fig 14 and 15.
Fig 14 - Microwave energy absorption as a function of electrical conductivity
Fig 15. Microwave interaction with materials. (x-axis represent dielectric loss factor) [61]
On the basis of microwave energy absorption characteristics, materials can be classified
into four principal groups [63-66]:
(i) Transparent: The low loss insulator materials through which microwaves pass without
being absorbed as characterized by the curve in category-1 (for example – Teflon,
quartz).

23. 17
(ii) Absorber: The high loss insulators, i.e. dielectric materials in which microwaves get
totally absorbed depending upon the value of the dielectric loss factor shown as the
category-2 materials are known as absorber materials (for example – water, SiC).
(iii) Opaque: The no loss insulators, i.e. conductor materials in which microwaves get
reflected without or negligible energy absorption, exhibit negligible skin depth and low
energy absorption characteristics as illustrated by the category-3 (for example – all bulk
metals).
(iv) Mixed absorbers: The advanced materials, i.e. composites or multi-phase materials
which have at least one of the phases as a high loss insulator, while the other(s) are low
loss insulator(s), this category of materials absorbs microwave by localized energy
conversion (for example – PMC, CMC, MMC).
The phenomena associated with the processing are less understood; popular mechanisms
such as dipolar heating and conduction heating have been mostly explored.
The absorbed microwave power in a material is the dissipated power due to the electric
and magnetic fields of microwave energy which could be represented as energy converted
inside a heated material. Thus, the power absorbed by a material is significantly influenced
by the depth up to which the radiations penetrate into it. However, microwaves cannot
penetrate inside in the similar fashion in all materials [61].
As the temperature of the material increases with time, all the material properties get
updated influencing the power absorbed.
The mass per unit volume affects dielectric and magnetic properties of materials during
microwave processing. It was reported that heating rate of lower green density copper
compacts was higher, while densification was more in higher green density copper
compacts during microwave sintering. The densification trends were, however, similar for
both- porous and high density copper compacts [67].
Walkiewicz et al. [68] studied the behavior of different metals at various heating rates
when exposed to microwave radiation and showed that metal powder coupled well with
microwaves, better than some dielectric metal oxides. Agrawal and his colleagues [69]
reported that a sheet of metal was reflected by microwaves, but in powder form, it seems
that metals are no longer so reflective.

24. 18
2.5.1.1 Heating mechanisms in microwave materials processing
Mechanism of heat generation during microwave–material interaction is complex. The
electric and magnetic field components of microwave agitate the orientation, position and
movement of dipoles, free electrons, domain wall and electron spin during material
processing. One or a combination of these phenomena do occur during the interaction
2.5.1.1.1 Heating mechanisms in non-magnetic materials
The non-magnetic materials are affected only by electric field component of microwave.
The two main loss mechanisms for non-magnetic materials (such as Al, Cu, water,
polymers, and ceramics) are dipolar losses and conduction losses. Conduction losses
dominate in metallic and high conductivity materials whereas dipolar losses dominate in
dielectric insulators.
Dipolar Loss - The dipolar loss is more effective in dielectric insulator materials in which
dipoles are generated when exposed to external electric field. These materials include
water, ceramics, CMC, PMC, food products. The process is shown below in Fig 16
Inertial, elastic, frictional and molecular interaction forces resist these frequent changes in
orientations of molecules which increase molecular kinetic energy and result in volumetric
heating. The kinetic energy increase of all dipoles in the material increases the temperature
of the material within a short time [70].
Fig 16 - Heating mechanism in dipolar loss.
Conduction Loss - The loss is significant in microwave processing of pure metals,
metallic based materials and semiconductors e.g. Cu, Al, Si, Fe, Ni, and MMC. These
materials have free electrons which starts movement in the direction of external electric

25. 19
field E with velocity v. The conductivity of these materials is significantly high;
consequently, the field gets attenuated rapidly inside the material which induces large
current (Ii). Hence, an induced magnetic field (Hi) is developed in the opposite direction of
external magnetic field inside the material. The induced magnetic field generates a force
on moving electrons that pushes conducting electrons in reverse direction with velocity vr.
Thus, a kinetic energy is imparted on electrons and movement of electron is restricted by
the inertial, elastic, frictional and molecular interaction forces. The oscillating electric field
repeats this phenomenon rapidly which generates volumetric and uniform heating inside
the material as shown schematically [71,72].
Fig 17 - Heating mechanism in conduction loss.
2.5.1.1.2 Heating mechanisms in magnetic materials
The heating mechanisms are typically active while microwave processing of magnetic
materials such as Iron, Nickel, and Cobalt. These materials are affected by both electric
field and magnetic field. The electric field imparts motion to the free electrons, whereas
the magnetic field affects the electron spin, domain wall and orientation of domains. The
heat loss mechanisms in magnetic materials exhibit conduction losses with additional
magnetic losses such as hysteresis, eddy current, domain wall resonance
and electron spin resonance [73].
2.5.1.2 Microwave Heating techniques and heat transfer modes
The widely-used heating techniques for processing material by microwave energy are
briefly discussed in this section considering a small volume of material.
2.5.1.2.1 Direct heating
The direct heating technique is used to heat materials which can be directly exposed to
microwaves, for example, ceramics, food products, metallic powders (Fig 18 a). In direct

26. 20
heating, microwaves easily couple with materials with heat generation inside the processed
material. The inherent temperature gradient during microwave processing causes
overheating of the material with formation of hotspots leading to thermal runway. Thermal
instabilities during processing of Al2O3, SiO2, Fe3O4, b-alumina, ZrO2, etc. by
this technique may cause non-uniform properties and cracking [73-75].
2.5.1.2.2 Selective heating
The selective heating technique is a special type of direct heating with certain constraints
as illustrated in Fig 18b. A special tooling is used for partial exposure of material to
microwaves for specific requirements such as in joining. The material to be processed is
covered with a masking material where exposure of material to microwave is not required.
The advantage of this technique is that the desired part of the material can be heated
without disturbing the properties in rest of the volume.
2.5.1.2.3. Hybrid heating
In order to overcome the problems associated with direct microwave heating, hybrid
heating technique (Fig 18c) was developed [76-78]. Later, this technique, also called
microwave hybrid heating (MHH), was successfully used for processing of materials such
as bulk metals which cannot be directly exposed to microwaves [79-82]. A special
arrangement, consists of susceptor (microwave absorber) and masking materials,
is required in this technique. The heating is completed in three steps – (i) the susceptor
heating, (ii) the conventional heating of the non-microwave absorbing material through the
hot susceptor (in step – (i)) and (iii) microwave heating of the target material
once the target material gets heated beyond its critical temperature in step (ii). The mask in
the step (ii), restricts the direct contact of microwaves with reflecting/non-microwave
absorbing material, whereas the susceptor couples with the microwaves and heats the
non-microwave absorbing material. Initially, the heat transfer from susceptor to material
takes place by conduction and convection; however, the radiation starts when the susceptor
temperature (TS) reaches beyond a critical value (TSR). The material absorbs energy form
susceptor until the temperature of material (TM) reaches a critical value (TC). Beyond the
critical temperature, the material absorbs microwave energy directly and rapid internal
heat generation does take place. In this phase, the heat flows from inside core to outside
surface of material.

27. 21
Fig 18 - Types of microwave heating (a) direct heating, (b) selective heating and (c) hybrid heating
Fig 19 - Bird's eye view of microwave materials processing

28. 22
2.5.2 Convention and Microwave based Material Processing
The microwave processing of materials in recent years had emerged as one of the novel
processing techniques that provides better processing characteristics such as lower
processing times, lower processing temperature, better microstructures, and enhanced
physical and mechanical properties in comparison with conventional routes. [83-85].
The direct absorption of microwaves to the atomic level of microwave-coupled materials
leads to volumetric heating of material from within the materials; this leads to rapid
heating rates with less thermal gradient inside processed materials. The rapid heating rates
arise due to volumetric heating characteristic of microwaves, which lowers down the
processing time and owing to which it consumes less energy in comparison with
conventional heating.
The microwave heating process involves the absorption of microwaves by materials
volumetrically and transfers this energy into heat by various phenomena of molecular
frictional heating, molecular polarization process, and resistance heating, whereas in
conventional heating, energy is transferred by conduction, radiation, and convection
phenomena. In the conventional process, heating is from outer surface to the inner core,
whereas in microwave heating, heat flows from core to outer surface as shown in Fig 20
below
Fig 20 - Heating mechanism for a) conventional Heating from outer to inner surface and (b)
microwave heating from inner to outer surface

29. 23
The challenge for researchers was to process bulk metallic materials at room temperature,
and it is very difficult because metals reflect microwaves and causes plasma formation.
The conventional heating of materials starts from the surface and heat transfer takes place
within the material with reduced temperature gradients. This can lead to the poor
microstructure of the surfaces, [86] and it may lead to the surface overheating or burning.
In contrary to conventional heating, microwave heating has inverse profile, that is, it starts
heating the material from within and transfers heat outwards as shown in Fig 20 above.
Microwave heating mode can lead to the poor microstructure of core, which can cause
thermal runaways, cracking, and burning of core. [87] To compensate the difference in
temperature gradients of surface and core, a new approach was used by researchers called
the two-directional heating or MHH. The principle of MHH is to operate on the
phenomenon of conventional heating and microwave heating concurrently, such that
heating of materials takes place from the outside as well as from the inside of materials.
The different heating phenomena are shown in Fig 22 below, which shows the
approximate flattening of temperature profile using MHH within the specimen. The MHH
produces uniform heating throughout the materials with reduced temperature gradients and
rapid heating. These characteristics are absent in conventional or microwave heating
processes.
The flattening of temperature profile reduces differential heating and can produce better
microstructures at cores as well as on surfaces. The initial heating of metallic powders by
conventional routes during MHH allows coupling of powders with microwaves at elevated
temperature, which helps in uniform heating and higher heating rates. MHH will be the
key to future developments in materials processing using microwave radiations.
Fig 21 - Comparison of heating procedure b/w microwave and conventional method [65]

30. 24
Fig 22 - Temperature distribution in conventional, microwave, and microwave hybrid heating [54]
Microwave technology is attractive because it has many obvious advantages when
compared with conventional methods, such as: very short cycle time resulting in energy
savings as high as 90% over conventional methods, rapid heating rates, finer
microstructures, and hence, improved mechanical properties and environmental
friendliness [59].

31. 25
Chapter 3
Plan of Work / Methodology
Metal Powder Foaming AgentSusceptor powder
Mixing these powders using tumbler mixer
Pelletizing using cold pressing
Conventional heat treatmentMicrowave heat treatment
Foaming of precursor material by heat treatment to its melting temperature
Characterization of the processed Sample

32. 26
Chapter 4
Experimental Procedure
4.1 Introduction
This chapter details the experimental setup and procedures used for production of
aluminium foams. When making foams one encounters several process variables which
have a lesser or greater influence on the foam characteristics. In this work, till now close
cell aluminium foam production using powder metallurgy route using titanium hydride
(TiH2) as the foaming agent. The green compact thus formed has then been heat treated
using microwave wave heating with different setup in order to generate and trap more heat
and also with conventional heating. Till now experiments has only been done with
aluminium and when succeeded in meta foaming with this material the process will be
extended to magnesium. Experiment has not initially been done with magnesium because
it is is highly reactive and less stable than aluminium also it is more expensive.
Characterisation of foam hasn’t been done since metal foam hasn’t been produce till now
though a hypothesis which is able to answer all to all the results has been inferred and is
been reported later in the report.
4.2 Materials Used
Powder metallurgy route for foaming has been adopted in our research work. For this the
material used has been reported in the table 2 shown below.
Table 2 – Acquired Material Property Table
Powders Manufacturer Mean Particle
Size (µm)
Purity
%
Melting / decomposition
Temp. (°C)
Al Alpha Chemica 99.7 660
SiC 37 2730
TiH2 Nanoshel 55 99.9 450
C (graphite) Alpha Chemica 99.9 3600 melting and auto
ignition at 730 °C
CaCO3 Alpha Chemica 98 825

33. 27
4.3 Procedure
The experiment to produce metal foam is done in two steps. First step deal with making a
green compact pellet then the next step deals with giving this green compact a heat
treatment.
4.3.1 Green Compact Preparation
For making a green compact first step is to get a rough idea about the final pellet shape and
size. Ten acquiring the die in order to get that shape and calculating the approximate mass
to be taken to get the desired height taking into account the apparent density of the
compact.
For my project, cylindrical shape of the pellet was decided and a die according to it was
acquired. For making a compact a composition was considered, pressure to be applied was
estimated to yield a final approximate porosity as according to curve shown below in Fig
23, a final weight of the pellet was roughly estimated to yield an approximate height and
finally by compacting the powder in die by hydraulic press i.e. cold uniaxial compaction.
Final compact having size of around 20mm diameter and 6-7 mm height were obtained
when a load of 200MPa was applied and it is estimated that this compact is having 5-10%
porosity. It is estimated that for there will be more porosity present when more amount of
silicon carbide will be present in aluminium matrix.
Fig 23 - Effect of Uniaxial Pressure on Green Density with powder size between 2-150 µm [88]

34. 28
Fig 24 - Final Sample Images after compaction
4.3.2 Heat Treatment
Samples were provided with either Microwave heating or conventional heating. The
equipment used for providing the heat treatment are shown in the table 2 and the
subsequent heat treatment given and the setup done for each sample processed is shown in
table 3.
For the microwave heat treatment among the available three equipment two were used and
further name for these applicators will be used on the basis of their power namely 900W
microwave, 1.4kW microwave and 3kW microwave. Till now 900W microwave and 3kW
microwave has been used. The experimental work has been planned to be done with third
applicator since it has IR pyrometer we will be obtain temperature data also the limitation
of microwave reflection toward filament to cause machine’s filament overheating is
believed will be less and therefore we will be able to provide microwave treatment for
longer time duration. But since it was not in the working state experimental work was done
with the alternative microwave applicators.
Conventional heat treatment was done using muffle furnace shown in the table 2.

35. 29
Table 3 - Equipment used for heat treatment
Heating
Method
Conventional Heating Microwave Heating Microwave Heating Microwave Heating
Equipment
used
Equipment
Rating
Operating Temp. Range: 0 -
1000°C
General Heating Rate: 6 - 8°C
Power Rating: 900 W
Frequency: 2.45 GHz
Power Rating: 3 kW
Frequency: 2.45 GHz
Power Rating: 1.4 kW
Frequency: 2.45 GHz
Equipment
setting
Time temperature cycle setting Timer Timer, IR Camera Timer, IR Pyrometer
Remark Temperature Sensor seems to
be inaccurate
Microwave filaments get heated within
10-20 min causing machine to
shutdown for an hour
Isn’t in working condition at the
moment

36. 30
Table 4 - Experimental setup, heat treatment given and final result for each sample
1. Sample (B1-1)
94.5% Al + 5% SiC + 0.5% TiH2
On decomposition of TiH2 it would have released around 9 ml of H2 gas.
3kW Microwave used
For a duration to 12 min **
Final Sample
2. Sample (B1-2)
94.5% Al + 5% SiC + 0.5% TiH2
On decomposition of TiH2 it would have released around 9 ml of H2 gas.
Muffle Furnace Used and Heat treatment given is as followed.
Temperature Time
RT  670 °C 2hr
At 670 °C 40 min
Final Sample
Insulator
IR Camera
Sample
Susceptor
Microwave Source
SiO2 Crucible
Boat Crucible
Sample
Muffle Furnace

37. 31
3. Sample (B1-3)
94.5% Al + 5% SiC + 0.5% TiH2
On decomposition of TiH2 it would have released around 9 ml of H2 gas.
Muffle Furnace Used and Heat treatment given is as followed.
Temperature Time
At 730 (10% more than the theoretical
melting point of Aluminium)
3hr
Final Sample
4. Sample (B1-4)
92.5% Al + 5% SiC + 2% CaCO3 + 0.5% TiH2
On decomposition of TiH2 it would have released around 9 ml of H2 gas.
Muffle Furnace Used and Heat treatment given is as followed.
Temperature Time
At 790 (10% more than the theoretical
melting point of Aluminium)
4hr
Final Sample
Boat Crucible
Sample
Muffle Furnace
Boat Crucible
Sample
Muffle Furnace

38. 32
5. Sample (B2-1)
74% Al + 25% SiC + 1% TiH2
On decomposition of TiH2 it would have released around 9 ml of H2 gas.
3kW Microwave used
For a duration to 16 min **
Final Sample
6. Sample (B2-2)
74% Al + 25% SiC + 1% TiH2
On decomposition of TiH2 it would have released around 9 ml of H2 gas.
3kW Microwave used
For a duration to 11 min **
Remark
3kW microwave got shutdown because of overheating and the sample didn’t
showcase any treatment over it had happened so it was the again treated in 900 W
microwave and considered as B2-3
Insulator
IR Camera
Sample
SiC Crucible
Microwave Source
Insulator
IR Camera
Sample
Graphite plate
Microwave Source
Graphite mould

39. 33
7. Sample (B2-3)
Sample B2-2 retreated here and since the sample didn’t seem to have undergone any
treatment before machine got shutdown it was again treated as an untreated sample.
74% Al + 25% SiC + 1% TiH2
On decomposition of TiH2 it would have released around 9 ml of H2 gas.
900 W Microwave used
For a duration to 32 min
Final Sample
8. Sample (B2-4)
100% Graphite
On decomposition of TiH2 it would have released around 9 ml of H2 gas.
3kW Microwave used
For a duration to 6 min **
Final Sample
Insulator
Sample
Graphite Mould
Microwave Source
Insulator
Sample
Microwave Source
Graphite Plate
Susceptor
Insulator

40. 34
9. Sample (B3-1)
74% Al + 25% SiC + 1% TiH2
On decomposition of TiH2 it would have released around 18 ml of H2 gas.
900W Microwave used
For a duration to 10 min
Final Sample
10. Sample (B3-2)
74% Al + 25% SiC + 1% TiH2
On decomposition of TiH2 it would have released around 18 ml of H2 gas.
900W Microwave used
For a duration to 10 min
Final Sample
Insulator
Sample
Susceptor
Microwave Source
Base Plate
Susceptor
Insulator
Sample
Microwave Source
SiO2 Crucible
Susceptor

41. 35
11. Sample (B3-3)
74% Al + 25% SiC + 1% TiH2
On decomposition of TiH2 it would have released around 18 ml of H2 gas.
3kW Microwave used
For a duration to 13 min***
Final Sample
12. Sample (B3-4)
74% Al + 25% SiC + 1% TiH2
On decomposition of TiH2 it would have released around 18 ml of H2 gas.
900W Microwave used
For a duration to 10 min
Final Sample
Insulator
Sample
IR Camera
Microwave Source
Insulator
Sample
Microwave Source
Graphite Plate
SiC Crucible
Insulator
SiC Crucible
Insulator

42. 36
13. Sample (B1-3)
97.5% Al + 2.5% TiH2
On decomposition of TiH2 it would have released around 44 ml of H2 gas.
Muffle Furnace Used and Heat treatment given is as followed.
Temperature Time
At 730 °C(10% more than the theoretical melting point of Aluminium) 2.5hr
730 °C  800 °C ≈ 0.5hr
At 800 °C ≈ 1hr
800 °C  900 °C ≈ 1 hr
900 °C ≈ 1hr
After observing that at 800 °C sample isn’t melting other aluminium based material
were also kept in the furnace and temperature was raised as shown above in the table
Final Sample
** Machine got shutdown because of filament overheating *** Process stopped because crucible got broke
Boat Crucible
Sample
Muffle Furnace

43. 37
4.4 Microstructure
One of the samples microstructure were observed.
4.4.1 Sample Preparation
Processed sample was cut in half using hex saw then belt polishing was done followed by
polishing by emery paper in the following way of grade – 320, 800, 1200 and 1500. After
this cloth polishing was done till nice finish was obtained. After cloth polish, chemical
etching was done using kellers reagent.
4.4.2 Microscopy
The prepared sample was then observed with Leica inverted microscope (fig 26) to
observed the microstructure. The microstructure result thus obtained for the sample B2-1
which had the composition of 74% Al + 25% SiC + 1% TiH2 are shown below in fig 25.
Fig 25 – Microstructure of final sample B2-1
Fig 26 – Leica Inverted Microscope

44. 38
Chapter 5
Results and Discussion
Following samples were prepared and the information about their composition is provided in table
4 below.

45. 39
Table 5 – Sample Treatment Description
Sample
Code
Sample Composition Heating Method Time
Period
Temp.
(°C)
B1-1 94.5% Al + 5% SiC + 0.5% TiH2 3kW Microwave 12min** -
B1-2 94.5% Al + 5% SiC + 0.5% TiH2 Muffle Furnace* ≈ 2.5 hr 670
B1-3 94.5% Al + 5% SiC + 0.5% TiH2 Muffle Furnace* ≈ 3 hr 730
B1-4 92.5% Al + 5% SiC + 2%CaCO3 +
0.5% TiH2
Muffle Furnace* ≈ 4hr 790
B2-1 94.5% Al + 5% SiC + 0.5% TiH2 3kW Microwave 16min** -
B2-2 94.5% Al + 5% SiC + 0.5% TiH2 3kW Microwave 11min** -
B2-3 94.5% Al + 5% SiC + 0.5% TiH2 900W Microwave 32min -
B2-4 100% Graphite 3kW Microwave 6min** -
B3-1 74% Al + 25% SiC + 1% TiH2 900W Microwave 10min -
B3-2 74% Al + 25% SiC + 1% TiH2 900W Microwave 10min -
B3-3 74% Al + 25% SiC + 1% TiH2 3kW Microwave 13min*** -
B3-4 74% Al + 25% SiC + 1% TiH2 900W Microwave 10min -
B4 97.5% Al + 2.5% TiH2 Muffle Furnace* ≈ 5hr 700 
800 
900
*  Muffle furnace temperature cycle has been described properly in Experiments section.
**  Equipment got turned-off b/c of filament over heating
***  Process stopped b/c of susceptor broke in b/w the process
Compaction Pressure for all samples was 200MPa
Setup for each sample treatment shown in Experiments
Observations
When Conventional material processing route was taken.
 It is evident that the muffle furnace was having error in temperature reading as
clearly sample B1-2, B1-3, B1-4 and B4 haven’t melt even when the temperature was
raised above melting point of metal matrix.
 Beads formation were observed on sample B1-2 and B1-3. Beads were formed mainly
at the corners and the average bead size with B1-3 was higher than with B1-2 without
the outliers. Which evident to larger bead will be formed when temperature will be
raised to a higher temperature with faster heating rate when the composition was
kept same.
 With sample B1-4 this was not observed and with sample B4 this phenomenon was
very limited even though this sample went to higher temperature compared to others.
(considering that furnace only had calibration error) indicating that composition
alteration indeed has effect of foaming (though this isn’t foaming exactly)

46. 40
 It is believed that bead formation is taking place because to the gas releasing from
the samples and encapsulating the melted surface metal and hence more beads
are observed the corners where there is more surface area available for gas to release.
 With sample B1-4 showing much less bead formation compared to B1-2 and B1-3
even when it was heat treated to higher temperature is indicating that the gas was
unable to be released this may be because pathways were not formed within the
porosity and gas was unable to be released and form beads. The above point also gets
reinforced from the fact that because of the presence of SiC within the matrix would
have resulted on more porous compact when compared to pure metal matrix. Hence it
can be said that porosity plays and important role in foaming and bead formation
was observed only because gas was able to create pathways to escape out
encapsulating the molten surface material.
 Observing all the evidence indicates that a proper heat treatment needs to be given
to the samples also less porous compacts are required for foaming to take place.
When Microwave material processing route was taken.
 It was observed with all the sample B1-1, B2-1, B2-2, B2-4 and B3-3 that because
high reflection happening within the chamber was causing the filament to get
overheated causing the 3kW microwave to shutdown automatically.
 Samples B2-3, B3-1, B3-2 and B3-4 were treated in 900W microwave in which though
we were able to treat sample for longer duration but still no result were obtained. This
indicates that 900W microwave power is not sufficient for our work.
 On sample B3-1, B3-2 and B3-3 treated through 900W microwave and also sample
B1-1 treated through 3kW microwave shows surface burning and taking in account the
susceptor positions it evident that susceptor position plays an important role.
 In all the sample treated through microwave it can be observed that surface texture is
more homogenous when treated with microwave as compared to conventional
based material processing.
 Point above also indicates that even when using hybrid heating in microwave our
material is interacting with microwaves and having inside-out heating.
 A trial was make to see if encapsulating our compact with in graphite would be
beneficial as microwave is observed to have better interaction with than graphite with

47. 41
metal. For this graphite compact was prepared, sample B2-4. It observed that with this
compact 3kW microwave had even smaller uptime and also compact temperature
didn’t raise much indicating that using graphite as an encapsulation to enhance
heating would have adverse effect with the formation of aluminium carbide and also
lesser microwave treatment period.
 With less porous pellet though our foaming characteristics will increase but compact
interaction with microwave will suffer.
 For all the samples treated with microwave, the effect of amount of SiC with in the
matrix isn’t well understood as our microwave treatment is getting limited because of
lesser power input by 900W microwave and with 3kW microwave getting shutdown
cause of overheating cause us to use susceptor i.e. hybrid heating for increasing the
kinetics of process. In this project, we are aiming to create foam by direct microwave
heating without the use of any other add-on like susceptor. So still we are waiting for
the 1.4kW microwave to get repaired which is also equipped with IR pyrometer for
temperature measurement.
TGA /DTA Result

48. 42
Chapter 6
Summary
Till now many sample had been prepared by providing either microwave or convention
heat treatment and some of the inferences observed from this are listed below-
 In bead formation porosity of compact played an important role. Bead formation took
place because the gas releasing because of decomposition created pathways within
porosity, escaping to the surface and encapsulating the molten surface metal. More
beads are observed at the corners because of more surface area for gas to release from.
Larger bead will be formed when temperature will be raised to a higher temperature
with faster heating rate when the composition was kept same also composition
alteration indeed has effect of foaming.
 Muffle furnace was having error in temperature reading and hence next time a proper
heat treatment needs to be given to the samples additionally less porous compacts are
required for foaming to take place. Though with less porous pellet, ease of foaming
will be more but compact interaction with microwave will suffer.
 We had to resort to using susceptor in microwave i.e. going for hybrid microwave
heating for increasing the kinetic of the reaction as our microwave treatment is getting
limited because of lesser power input by 900W microwave and with 3kW microwave
getting shutdown because of filament overheating caused by large reflection taking
place in microwave chamber.
 Surface texture were observed to be more homogenous when treated with microwave
as compared to conventional based material processing even when using hybrid
heating in microwave our material was interacting with microwaves and having inside-
out heating.
 Using graphite as an encapsulation to enhance heating would have adverse effect.

49. 43
To finally conclude in this project, we are aiming to create foam by direct microwave
heating without the use of any other add-on like susceptor. So still we are waiting for the
1.4kW microwave to get repaired which is also equipped with IR pyrometer for
temperature measurement. Additionally, our muffle furnace was having some error which
will be resolved and the next batch of sample will be prepared with more load. In future,
we will need to balance to of porosity as more porosity will lead to better interaction of
microwave with the compact while more porosity will lead to evolution of gas hence metal
foaming will suffer.

50. 44
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