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CHAPTER 1

INTRODUCTION

1.1 GENERAL

In recent years, there is a high demand for the development of light-

weight metal foams. In past, any kind of pores in the metal was considered as

“defect”, but now the same class of defective material with pores is getting

importance and the metal is termed as “metal foams” or “porous metals”.

Metal foams have excellent physical and mechanical properties. According to

Banhart and Baumeister (1998) a pore is an open volume within the metal

matrix or network with uniform distribution and length of passages. By

manipulation of process parameters, the pore structures can assume

continuous or discontinuous geometries. Metal foams include small filaments

that are continuously connected in an open-celled foam structure. Metal foam

cells are usually polyhedrons of 12-14 faces in which each has a pentagonal

or hexagonal shape. The pore size is one of the most important characteristic.

The pore density is the number of pores that can be measured in

linear inch and its unit is PPI (pores per inch). The strength of the foam

depends mainly on the base material and relative density of the foam. Other

properties, such as pore size, pore density, area density and cell shape affect

certain foam characteristics. Pore size and relative density affect foam’s

flexibility. The pore size is specified by the diameter of the open space in

each of the cell faces. The relative density is the volume of solid foam

material relative to the total volume of metal foam.

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Metal foam possesses novel properties such as low density, high

specific stiffness, high strength to weight ratios, and greatly increased energy

absorbing capabilities. Applications include automobile crumple-zones,

automotive structural components, biomedical prosthesis, ballistic armour,

sound barriers and vibration dampeners. According to Ashby et al (2000)

metal foams are classified into open cell and closed cell metal foams.

In open cell metal foams, pores are continuous and connected

together so that, the fluid flows from one side to the other. Open cell foams

find application in filtering and also in heat exchangers (Shadi Mahjoob and

Kambiz Vafai 2008).

Srivastava and Sahoo (2006) states that the closed cell foams are

being used in light-weighted constructions due to their high stiffness and low

density. Closed cell configuration is optimal for energy absorption. Closed

cell metal foams are also used as sound dampers and they are placed in

automobile fire wall material and buildings (Lu 2002).

Due to growing consumer demands and stiff competition, the

present day developing industries are induced to produce low weight products

with low cost. Porous metal components are designed with required properties

for specific applications. Porous metals have raised interest and importance in

the last two decades and started to find their way in many research and

industrial applications.

The content of this thesis divided into 3 chapters.

Chapter 1 introduces the subject and contains the objectives of this

research. This chapter also contains a detailed and extensive literature review.

Applications of metal foams, metal foam evolution and various production

methods of metal foams were discussed in detail. Production methods of both

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liquid metallurgy route and powder metallurgy route were also discussed in

detail.

Chapter 2 deals with experimental work. Experiments were

conducted to produce porous structures on gunmetal, stainless steel and

aluminium castings. Production method consisting of layout, with the

preparation of sand balls, then follows with other steps systematically as

preparation of mould, melting of metal, pouring, knockout, cleaning and other

steps. The characterisations of porous castings were also carried out.

The data on results and discussion of this research work is given in

Chapter 3. Testing of porous gunmetal castings include visual examination,

radiography test, density measurement, porosity measurement, visual

examination of cut-section, compression test and hardness test. Also visual

examinations, radiography tests, density measurements and porosity

measurements of both stainless steel and aluminium porous castings are also

presented. The conclusions of this research and suggestions for future work

are also presented in this chapter.

The goal of this research work is to develop metal foams that show

improvements in mechanical properties and product uniformity. To attain this

goal, the study included the identification of various techniques used to

manufacture porous castings and focused on improvements to meet the goal

and the porous samples were produced successfully. Using these samples a

series of characterisation studies were carried out to qualify and quantify the

results. These findings were then compared to presently published data to

gauge the relative success of the work.

The porous castings developed in this study displayed significant

improvements in the measures of compressive strength and also maintaining

the physical and mechanical properties of cellular metals. Several areas for

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improvements have been identified for this technology. The method of

analysis will continue to improve this product to satisfy the objectives of this

research program.

1.2 OBJECTIVES OF THE RESEARCH

1. The main objective of the present Research is to produce

porous gunmetal molten from oil fired crucible furnace, to

produce porous stainless steel molten from electric induction

furnace and to produce porous aluminium molten from electric

resistance furnace.

2. To find out the percentage porosity by varying the size of the

pores.

3. To make use of sand and Bentonite for creating porosity.

4. To analyze the porous castings by density, the percentage

porosity determined in gunmetal, stainless steel and

aluminium.

5. To study the internal nature of the pores by Radiographic

testing on gunmetal, stainless steel and aluminium castings.

6. To study the internal nature of the pores by cut-section

analysis on porous gunmetal castings.

7. To study the mechanical properties of porous metal by

compression testing and Hardness tests on gunmetal castings.

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1.3 LITERATURE REVIEW

1.3.1 Introduction

Metal foams are a new class of materials of great interest due to

their unique combination of properties derived from their cellular structure

and metallic behaviour (John Banhart 2001). Liu et al (2006) stated that the

metal foam cells are usually polyhedrons of 12-14 faces in which each face

has a pentagonal or hexagonal shape (by five or six filaments). Kunze et al

(1993) stated that the porous castings form a subgroup of cellular metals,

usually having polyhedral cells but shapes may vary in cases where

directional solidification creates different morphologies.

There are two major types of porous structures as open cells and

closed cells or combination of the two. They may be either closed with

membranes separating adjoining cells or open if there are no membranes

across the faces of cells so that the voids are interconnected. Khayargoli et al

(2004) reported that open pore generally have a cellular structure made up of

3-dimensional interconnected network of solid plates that form the edges and

faces of the cells. Babscan et al (2006) states that, the solid foams originating

from liquid foam are closed. Ashby et al (2000) stated that the characteristic

properties define porous structures which include its cellular structure and

relative density. Open cell can be thought of as a network interconnected solid

struts. Wadley (2002) states that, a combination of open cell and closed cell

composition is technologically possible.

John Banhart and Denis Weaire (2002) reported that Benjamin

Sosnick in 1943 attempted to foam aluminium with mercury. First, mixture of

aluminium and mercury was melted in a closed chamber under high pressure.

The pressure was released, which led to the vaporization of the mercury at the

melting temperature of aluminium that led to formation of foam. When

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speaking of porous castings or porous structures or metal foams, one

generally mean only solid foams. Ashby et al (2000) stated that porous

castings are also called “foams” although the term “sponge” is more

appropriate.

Ashby et al (2000) also reported that in comparison to solid metals,

metal foams offer high specific stiffness; stiffness to weight ratio and the

required mechanical properties can be arrived for wide range of applications

by altering the size, shape and volume fraction of cells. Babcsan et al (2006)

stated that the development of porous structures have improved the properties

when compared to non-metal foams. Metal foams offer high stiffness,

increased impact energy absorption and better strength to weight ratios. San

Marchi et al (2004) reported that the tensile failure strains are relatively low

in foam compared to the bulk material from which they are made.

Yves Conde et al (2006) reported that the grading of porosity in a

bent metal skin/metal foam core sandwich can generate significant weight up

to 15 percent reduction in total sandwich beam mass at equal allowable load.

It will be best to keep density of foam to its lowest possible value because the

modulus decreases faster with decreasing foam density.

Jerzy Sobczak (2003) states that the liquid metallic foam is merely a

stage that occurs during fabrication of the material. Coxa et al (2001) stated

that the surface energy is minimized in liquid and allows only for certain

morphologies. The solid foam, which is just an image of its liquid

counterpart, is restricted in the same way.

Mc Cullough et al (1999) reported that the behaviour of closed cell

aluminium foam (Alulight) behaves in a semi-brittle fashion on tension and

ductile behaviour on compression. Yield strength and unloading modulus are

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equal in tension and compression, and increase non-linearity with relative

density. Addition of low silicon content produces stiffer and stronger foam.

Lotus-type porous stainless steel possessing cylindrical pores

aligned in one direction was developed by a continuous zone melting

technique in a pressurized mixture gas of hydrogen and helium. Compression

tests were carried out not only in directions parallel and perpendicular to the

elongated-pore direction but also in other directions to reveal its anisotropic

compressive behaviour (Tane Masakazu and Nakajima Hideo 2006).

Dispersion of one phase into a second one, each phase can be in one

of the three states of matter shown in Figure 1.1.

Figure 1.1 Dispersions of one phase into a second phase can be in one

of the three states of matter (John Banhart 1999)

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Jaroslav Kovacik and Frantisek Simancik (1999) investigated and

compared the usage of aluminium and zinc foams to enhance stiffness and

increase energy absorption at reduced weight for automotive components.

Attention was given to the heating temperature.

The following are the important process parameters that control the

pore formation in the castings.

SIZE OF CORES: Kan-Sen Chou and Ming-An Song (2002)

stated that the final pore structure can be manipulated by the

ball size and the load applied during packing the balls. The

shape and size of the pores directly depends on the cores used.

TEMPERATURE: Lakshmi et al (2007) reports that the

mechanical behaviour of the foam depends on the processing

temperature. Properly controlling the holding temperature

leads to the production of foams which contains a uniform cell

structure and high porosity (Yang and Nakae 2000). Soong-

Keun Hyun et al (2004) pointed out to maintain the pouring

temperature of the molten metal till the cavities or voids are

filled. Solidification at intermediate layer may result

insufficient metals filling of cavity.

VOLUME: When the volume of the core used is maximum,

the interconnectivity of pore is more and less amount of metal

is required to fill the voids. Minimum volume of core used in

relation to volume of metal results in formation of closed cell.

The inorganic cores cannot be removed fully resulting in

production of less porosity (John Banhart and Denis Weaire

2002).

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PREHEATING: Babscan et al (2006) reports that the cores

and the die are to be preheated to remove the moisture and

also to avoid rapid solidification. Sudden cooling can lead to

deformation of the films or create cracks in cell walls (John

Banhart and Denis Weaire 2002).

Porous castings have wide range of relative densities spanning from

2% to 100%. The application of porous castings decides the material from

which it is to be processed. The size and distribution of pores in metal matrix

are random. The properties of metallic foams are evaluated according to

apparent density (Frantisek Simancik 2001). Figure 1.2 shows closed cell

metal foam manufactured by Franhofer-Institute in Bremen, Germany

(Banhart and Baumeister 1998).

Figure 1.2 Closed cell foam produced by Franhofer-Institute in Bremen,

Germany

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Rabiei and Vendra (2009) investigated on composite foams and

reported that the composite foams exhibit ultra high-strength when handling

large densification strains above 50% absorbs 7-10 times than any other metal

foams. Also the composite metal foams deformed uniformly under both

monotonic and cyclic compression loading without any collapse bend

formation. Lakshmi et al (2007) reported that the developed composite metal

foam for the first time using gravity casting technique and characterised using

monotonic compression. Addition of a solid matrix to hollow sphere foam

resulted in structural integrity, strength and increased stability. Composite

metal foam displays the characteristic properties of elastic-plastic foam in

compression, with linearly elastic region at initial stage, and followed by a

long plastic deformation. Also the mechanical behaviour was found to be

dependent on processing temperature.

1.3.2 Applications of Metal Foams

Srivastava and Sahoo (2006) summarize and report that the metal

foams recently considered as a revolutionary material due to their unique

combination of physical and mechanical properties such as high stiffness, low

specific weight, high gas permeability, low thermal conductivity, unusual

acoustic properties, high impact absorption capacity and good electrical

insulating properties.

Metal foams are identified as new class of materials of great interest

due to their unique combination of properties derived from their cellular

structure and mechanical behaviour. Metallic foams find application in

structural and automobile industry. They also find functional applications in

filters, heat exchangers, silencers, flame arresters and also in water

purification (John Banhart 2001). Metal foams convert impact energy into

plastic work and absorbs more energy than its bulk metal (Kelly 2006).

Common applications of metal foam are listed in Table 1.1.

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Table 1.1 Lists the applications of metal foam

Applications for openand closed cellmetallic foam

Comment

Lightweight structures Excellent stiffness-to-weight ratio when loaded in bending.Sandwich cores Metal foams have low density with good shear and fracture

strength.Strain isolation 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 a factor of 10.Vibration control Foamed panels have higher natural flexural vibration

frequencies of solid sheet of the same mass per unit area.Acoustic vibration Reticulated metal foams have sound absorbing capacity.Energy Management Metal foams have exceptional ability to absorb energy at

almost constant pressure.Packing with hightemperature capability

Ability to absorb impact at constant load, coupled withthermal stability above room temperature.

Artificial wood Metal foams have some wood-like characteristics: light,stiff, and ability to be joined with wood screws.

Thermal management Open-cell metal foams have large accessible surface areaand high cell-wall conduction giving exceptional heattransfer.

Heat shields Metal foams are non-flammable; oxidation of cell faces ofclosed-cell aluminum foams appears to impart exceptionalresistance to direct flame.

Consumable cores forcastings

Metfoams, injection-molded to complex shapes, are used asconsumable cores for aluminium castings.

Biocompatible inserts The cellular texture of biocompatible metal foams such astitanium stimulates cell growth.

Filters Open-cell metal foams with controlled pore size havepotential for high-temperature gas and fluid filtration.

Electrical screening Good electrical conduction, mechanical strength and lowdensity make metal foams attractive for screening.

Electrodes and catalystcarriers

High surface/volume ratio allows compact electrodes withhigh reaction surface area.

Buoyancy Low density and good corrosion resistance suggest possiblefloatation applications.

(Source: Ashby et al 2000)

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Srivastava and Sahoo (2007) reported that the National Physical

Laboratory (UK) conducted a survey on both in industries and in research

institutions to have an idea of potential expectations from metallic foams.

Figure 1.3 represents the projected potential applications in various fields.

Around 32% of future requirements are projected for automobile and

aerospace industries.

Transport 26%

Other industries18%

Resea rch /education 16%

Componentmanufacturing

11%

Ma terialsma nufacturing

10%

Aerospa ce 6%

Engineeringmanufacturing

5%

Powerengineeing 5% Process

industries 3%

Figure 1.3 Projected applications of foams for industrial sectors

In open-cell configuration, the pores are open and allow the passage

of fluids and gases to pass through them. Metal foams having open cells are

said to have sound absorbing capacity. They have higher damping capacity

and natural vibration frequencies than a solid base material (Lu et al 1999).

Open-cell metal foams find additional applications in the field of heat

exchangers, filters and catalyst carriers.

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Figure 1.4 shows the open-cell metallic foams. Cells are

interconnected through open faces.

Figure 1.4 Open-cell configurations

In closed-cell configuration the pores are not interconnected and do

not allow the passage of fluid and gases. Kunze et al (1993) reported that the

closed-cell metallic foams are well suited for use as floating structures

because of their high damage tolerance. These structures retain their buoyancy

even when locally damaged. Closed-cell metal foams withstand high pressure

and temperatures. Ashby and Tinjian (2003) reported that the metal foams

predominantly closed cells are poor sound absorbers. Figure 1.5 represents

closed-cell in which the faces of cell are sealed.

Figure 1.5 Closed-cell configurations

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Hideo Nakajima (2007) reported that sound absorption materials

with an advanced performance to noises are required for mufflers of cars, air-

conditioner parts and pump chambers. The glass wool and foamed aluminium

with closed pores are frequently used as sound absorbing materials. But these

materials have low strength though they have good sound absorbing capacity.

Lu et al (1999) stated that the foams having low initial relative densities are

better sound absorbers than those with high relative densities. Selected metal

foams exhibit sound absorption co-efficient between 80% and 95% in selected

frequency ranges. Sound absorption means an incident sound wave neither

reflected nor transmitted, but the energy absorbed in the material. Compared

with wool and polymer foams, metal foams are excellent sound absorbers due

to rigidness, strength, fire retardance, low moisture absorption and exhibit

superior impact energy absorption capabilities.

John Banhart and Denis Weaire (2002) reported that the wide range

of application of foams materials in automotive, aerospace, railway, nautical,

civil engineering and medical fields, due to high-stiffness to weight ratio and

vibration damping capacity. Ship building industries could utilize large panels

of aluminium foams for doors and escape hatches. Conventional dense metals

could also be replaced in industries by foam filled columns or sandwich

panels in order to reduce their inertia and to damp vibrations. This can be

done by making replacements in printing rolls or quickly moving platforms or

crossbeams in machines. Biomedical industry could also focus on foam based

titanium as dental implants, since titanium is biocompatible and by selection

of appropriate porosities and elastic properties, foam also can be adopted to

the modulus of bones. Lu et al (2002) reported that the sandwich panels with

metal foam cores offer significant potential for acoustic and vibration control.

Sintering dissolution process is capable of producing low cost, net

shaped aluminium foams with controlled morphology, size distribution and

porosity. The process stood promising for manufacturing aluminium foams

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with relative densities in the range of 0.15- 0.5 and has potential for industrial

applications (Zhao and Sun 2001).

Vaziri et al (2006) investigated the role of low-density structural

polymeric foams filling the interstices of the cores of metal sandwich plates to

ascertain the strength of the cores and the enhancement of the plate

performance under crushing and impulsive loads. Also informs that sandwich

plates with foam-filled square honeycomb cores and folded plate cores exhibit

comparable structural performance in raising deformation to sandwich plates

of equal mass with unfilled cores under quasi-static and impulsive loads.

John Banhart and Denis Weaire (2002) reported that the German

automobile company Wilhelm Karmann in collaboration with the Fraunhofer

Institute in Bremen has developed a foam sandwich technology shown in

Figure 1.6. A flat sandwich panel with two faces sheets of aluminum with a

foamed aluminum in the inner core. Such components are damage tolerant

and easy to integrate into a car’s body.

Figure 1.6 Lightweight aluminum foam sandwich

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Metal foam materials have the potential to increase heat transfer

rates from solid surfaces by conducting heat to the material struts and

inducing a high interaction between the struts and a through flowing liquid.

Under identical conditions metal foam heat exchangers have

improved advantages than commercially available heat exchangers. Metal

foams provide larger heat transfer rates due to more heat transfer surface area,

more boundary layer disruption and mixing resulting from foam filaments.

Structural properties of metal foam’s, such as pore size, pore density, relative

density and porosity control the heat transfer processes (Shadi Mahjoob and

Kambiz Vafai 2008). Lu et al (1998) reported that the metal foams posses a

range of thermo chemical properties that suggest their application in areas

demanding impact/blast amelioration, heat dissipation, acoustic isolation and

heat exchangers. Figure 1.7 shows a heat exchanger prototype manufactured

by Porvair.

Figure 1.7 Heat exchanger prototype manufactured by Porvair

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Kunze et al (1993) developed and reported that hollow profiles

made of bulk metal can be filled with foam, giving rise to better deformation

behaviour of the parts during loading. Inner parts of the car, like engine parts

can be made or reinforced with foamed metals in order to gain higher stiffness

combained with a net weight savings. Figure 1.8 shows a structural foam part

made of highly porous inner aluminium core and a dense outer part.

Figure 1.8 Structural metal with inner aluminium foam

John Banhart and Denis Weaire (2002) reported that the metal

foams collapse gradually under critical compressive load until a high degree

of compaction is achieved. Due to this the metal foams absorb great deal of

mechanical energy and have a high yield stress compared to polymer foams.

Metal foams are considered sacrificial elements of modern vehicle design that

are deliberately intended to collapse in order to save passengers. The foam

material might be determined as heavy duty polystyrene because one cubic

centimeter of aluminum foam can absorb up to 10 joules of mechanical

energy if crushed to 20% of its original length.

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Srivastava and Sahoo (2007) stated that an attempt was made to

analyze an idea of the frequent accidental possibility in different parts of a

car. Also transport industry are basically intended in weight saving, impact

absorbing and thermal insulation. Figure 1.9 shows a sample car designed by

Karmann Germany with components made of aluminium foam. It is clearly

indicated that 67% of in-vehicle injury cost occur during collision at the front

end and 22% during collision from the side. Therefore utilization of foamed

material becomes necessary for passengers safety. The imperative should be

to use metallic foams in front end for passenger safety and rear end to reduce

the weight. The difference between these two applications will be based on

the foams quality to be used.

Figure 1.9 Karmann car with parts made of aluminium (Courtesy:

IFAM, Bremen, Germany)

Rachedi and Chikh (2001) reported to have adopted a new technique

by insertion of foam or porous material between the components on a

horizontal board to enhance heat transfer and improve cooling performance.

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Porous material insertion allowed a decrease in the maximum temperature in

the components according to its permeability and thermal conductivity. The

maximum temperature was reduced up to 50 percent.

Boomsma and Poulikakos (2001) reported that one-dimensional heat

conduction model use with open-celled metallic foam was developed based

on a three-dimensional description of the foam geometry. The three-

dimensional model demonstrated for the metal foam, in which the solid

conductivity is higher than fluid conductivity. By increasing the thermal

conductivity of the solid phase through manipulation of the solid structure at

the manufacturing phase, the overall thermal conductivity is improved.

Yi Feng et al (2002) reported to have developed aluminium alloy

foams with different densities and cell diameters with powder metallurgy

technique to examine electrical conductivity. It was clearly noticed that the

electrical conductivity depends on foams relative density, where as the cell

diameter appears to have negligible effect.

Ziya Esen et al (2009) reported that the alloy of porous titanium and

Ti6Al4V, manufactured by sintering the powders at various temperatures in

loose conditions finds applications in biomedical fields for use in orthopedic

and dental implants. The alloy samples had moduli of elasticity equivalent to

human bones modules, yield strength were equivalent to cancellous bone

strength and yield strength of alloy samples were comparable to those of

cortical bone. Purvi SD Patel et al (2008) investigated and presented the use

of closed cell polyurethane foams as standard test material for mimicking

human cancellous bone and fixation of bone screws.

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1.3.3 Metal Foam Evolution

The foam evolution can be of two types, namely internal evolution

and external evolution. In the former case gas bubbles are created by gas

evolution with in the melt. Gas evolutions occur due to the decomposition of

foaming agent when heated to its melting temperature. Park and Nutt (2000)

reported that the heating temperature and holding time are the major

parameters for evolution. Mostly hydrides or carbonates used as blowing

agent. Creation of water vapour in the melt due to chemical reaction acts as

the driving force for expansion of the foam.

In external evolution, bubble creation is caused by injection of gas

in to the melt from outside through a capillary or a porous layer (Losito

2008). Internal and external evolutions adopt different evolution phenomena

during foaming. In former case bubble evolution travels certain distance in the

melt, where as in the later case restricted to injection point to the surface.

How much and how good foam can be produced from a liquid,

quantifies the term formability. Ashby et al (2000) stated that the volume

fraction of foaming agent ultimately determine the relative density. The cell

size can be determined by the volume fraction of blowing agent and cooling

condition.

Ashby and Tianjian (2003) stated that the formation of foam until to

collapse is termed as foam evolution. The evolution of foam is of three stages,

first stage is heating, continued by holding stage (isothermal) and finally the

cooling stage. The stages of evolution of foam represented in the Figure 1.10

and Figure 1.11. Figure 1.10 represents the foams made by foaming

precursors and Figure 1.11 represents the foams made by external gas

evolution.

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Figure 1.10 Foams produced by foaming precursors

Figure 1.11 Foams produced by external gas injection

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Yang and Nakae (2000) stated that the expansion is governed by the

thermal decomposition reaction (hydrogen). At optimum temperature

sufficient foaming kinetics occur due to pressure increase and release of gas is

accelerated, which causes bubble coalescence very rapidly. The final bubble

size and the total volume of porosity related to the gas content in the melt is

specified by the growth rate between a solid-liquid interface. If the holding

temperature is less, sufficient foam release does not occur resulting in smaller

cell sizes and presence of solid phases (due to insufficient hydrogen in melt).

If the holding temperature is high excess gas released from the melt leads to

production of unstable liquid foam. So optimum holding temperature can be

maintained for better results.

Zhao et al (2005) reported to have foamed copper by using

potassium carbonate as decomposing agent. Evolution of carbonate is

essential and succeeded by maintaining optimum temperature (maintaining

temperature above melting point of carbonate) and maintaining right holding

time. The resultant copper foam was very clean and porosity of 80%

achieved.

Davis et al (2001) reported that the titanium-based foams have

excellent potential applications due to titanium’s outstanding mechanical

properties, low density and high chemical resistance. Solid-state foaming of

commercial purity titanium was achieved by pressing of titanium powders in

the presence of argon gas, followed by expansion of argon bubbles at ambient

pressure and temperature.

1.3.4 Production Methods of Metal Foams

1.3.4.1 Liquid metallurgy route

Pure metal is not fomable directly, so it is necessary to modify the

melt. In this method foam is created by either addition of reactant and

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foaming agent (distributing a gas release agent like metal hydride) or by

blowing gas or air in to the molten metal, so that the foam metal is formed.

The foaming metal can be solidified by cooling in a foaming vessel or can be

drawn off melt surface to solidify on a separate conveyer belt.

1.3.4.1(a) Melt gas injection

John Banhart and Denis Weaire (2002) reported that the Canadian

company Cymat is industrializing this process. In this Hydro/ Alcon process

liquid metals and their alloys can be foamed directly by injection of gas like

argon or silicon carbide in to the melt in a separate chamber through a

specially designed injector and stirred simultaneously and drawn off through a

conveyor belt continuously from the surface of melt with required cross

section and length. Figure 1.12 showing direct foaming of liquid melt by gas

injection.

Figure 1.12 Direct foaming of liquid melt by gas injection

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Losito (2008) reported that the gas injection is the most adopted

process of foam realization, in which variety of gases can be used to create

gas bubbles in the liquid metal. Low relative density of closed-cell foams can

be produced by carefully controlling the gas injection and cooling rate.

1.3.4.1(b)Direct foaming of liquid melt by gas injection

In this method foaming melts are directly added with foaming agent.

The foaming decomposes under the influence of heat and releases gas which

accelerates the foaming process. TiH2 and ZrH2 serves as a foaming agent

releasing gas when it is heated (Banhart and Baumeister 1998). When

foaming is complete the melt is cooled to solidify the foam before the gas

escapes and the bubbles coalesce or collapse. The pore structured named

“ALPORAS” and the method developed by Shinko wire company.

Figure 1.13 shows the process steps involved in manufacture of Alporas.

Figure 1.13 Process steps involved in manufacture of alporas by

Tetsuji Miyoshi et al (2000)

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1.3.4.1(c) Casting around space holder materials

Casting around space holder technique practiced in this research

work. According to Banhart and Baumeister (1998) light weighted porous

castings can be produced by casting around organic or inorganic granules.

Figure 1.14 shows the steps involved in this technique.

Figure 1.14 Making metallic foams using filler material

This technique is partially economical. It is manufacturing process

by which molten metal poured into a cavity having hollow cavity of desired

shape. The granules are introduced into the melt or the melt is poured over the

filler material and allowed to solidify. The heat capacity of the granules is

very low and therefore it does not disturb the flow of metal too much. The

filler material is removed after solidification of metal. The casting of granules

around space holder technique produce interconnected cell structures.

Ken-sen Chou and Ming-An Song (2002) reported to have produced

porosity of 88.5% by using casting technique. Ceramic balls were used as

filler material. The spherical shape was desirable to the regular packing of

ceramic balls in the mould. Figure 1.15 shows the ceramic balls.

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Figure 1.15 Picture showing ceramic balls

1.3.4.1(d) Spray forming

Spray forming also called “Osprey process”. It is a method for

processing variety of metals and alloys. In this method metallic melt is

continuously atomized and sprays of fast flying metal droplets are created.

The droplets are collected on a substrate and they grow to a dense deposit in a

given shape (billet or tube), when process parameters are rightly choosen.

Low oxide content, fine grain size or high metastable alloy phases are the

main characters of spray formed materials. These excellent properties cannot

be achieved by conventional casting methods.

For modifying the properties of the deposit by injecting powders

such as oxides, carbides or pure metals in the spray allowing them to react or

be wetted by liquid metal droplets and to incorporate in to the metal

deposition on the substrate makes the spray forming method attractive (John

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Banhart 2001). Figure 1.16 represents the process steps for making syntactic

foams.

Figure 1.16 Process steps for making syntactic foams

1.3.4.1(e) Gas entrapment technique

Martin (1996) reported that the method of processing combine the

affordability and structural stability of conventional solid metal with ultimate

weight efficiency of conventional sandwich structure. In this method a shell

container is initially prepared from a solid material. The container is sealed

after filling with a reactively compatible metal core material and a gas. The

sealed gas filled and core filled shell container is subjected to a heat treatment

sufficient to convert the same in to a consolidated metal billet in which the

gas is trapped with in the metal core, without melting the metal shell container

or core.

The gas filled consolidated billet is an intermediate product which is

then treated like a solid metal mill working technology by deformation-

processing and cut to shaped billets having any predetermined shape. Thus,

the obtained billet is heated to expand the gas trapped in its core to create an

internal network of pores or channels and produce structural porous metal

element having an integral sandwich type structure. The structure is

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characterised by a low density metal core and a solid metal facing. The light-

weighted products are well suitable for space industry, aircraft industry and

ship building industry.

Douglas et al (2001) reported that the light-weight structure

developed by entrapped gas expansion process, find application in aircraft

door, wing and stiffer skins. It was observed that maximum attainable

porosity was limited by the loss of gas pressure, which occurs as a result of

gas lost through the external surface of the expanding body.

John Banhart (2001) reported that the densified material can finally

be worked in to a near-net shape and converted in to a cellular material by

means of appropriate annealing temperature. This annealing step takes place

at 0.6 times the melting temperature of respective alloy and takes 6-24 hours.

During annealing the gas pores slowly expand and lowering their internal

pressure until equilibrium between the gas pressure and the strength of the

metal at annealing temperature has been reached. Figure 1.17 shows the steps

involved in gas entrapment technique.

Figure 1.17 Gas entrapment technique (Martin 1996)

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1.3.4.2 Powder metallurgy route

Banhart and Baumeister (1998) stated that the metal powders can be

used to make porous metallic structures. In some process the powders are

processed into a compact precursor material prior to actual foaming, in others

powders are used for direct foaming.

Baumeister et al (1994) stated that the common method for

production of porous materials with an open or interconnected porosity uses

sintering of loose powders. The powders are filled in to a mould and sintered

under conditions which provide maximum amount of porosity. Relative

densities in the range of 40 to 60% can be attained depending on size and

shape of the metal particles used. Higher degree of porosity also can be

achieved by using spacing agents that can be removed during or after the

sintering process.

Murray and Dunand (2003) reported that the solid state foaming of

pure titanium was achieved by powder metallurgy technique. Under

isothermal foaming conditions, the pores remained relatively small and

spheroid in shape. Foaming under thermal conditions pores grew much faster

than isothermal condition and also the porosity was higher.

Orinakova et al (2004) reported that the advantages of copper-nickel

coatings on porous hollow iron particles. Cellular materials offer attraction for

the production of light-weight components by powder metallurgy route. The

hollow iron spheres are substances suitable for preparation of regularly

structured cellular materials. By applying metallic-nickel coatings the

properties of the original hollow properties can be modified.

Sanjay R. Arwade et al (2011) investigated and reported variety of

methods for developing steel foams for use in civil structural applications.

Also informs that powder metallurgy-hollow spheres and composite powder

metallurgy-hollow sphere process find advantageous.

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1.3.4.2(a) Blowing agent as gas source

Srivastava and Sahoo (2007) reported that the powder compact

melting process comprising blending of metal or alloy powder with the

foaming agent, compaction of powder blend, deforming or working and

foaming. Compaction of powders can be accomplished by cold working,

sintering, hot pressing, powder rolling and powder extrusion. The

fundamental aim in foaming step is to form a very dense foamable precursor

with uniform distribution of the embedded blowing agent without any residual

open porosity. A schematic process is shown in Figure 1.18.

Figure 1.18 Processing steps involved in powdered metal foaming with a

blowing agent

Banhart and Baumeister (1998) stated that the heat treatment at

temperatures near to the melting point of the matrix material. During this

process the foaming agent which is homogeneously distributed with in the

dense metallic matrix decomposes. The gas released forces the material to

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expand forming highly porous structure. Before foaming the precursor, the

material can be processed in to sheets, rods or profiles of required shape by

conventional techniques like rolling, sawing or extrusion in order to improve

the flow conditions during foaming inside moulds. The density of metal

foams can be controlled by adjusting the content of foaming agent and several

other parameters such as temperature and heating rates.

1.3.4.2(b) Foaming by generation

In this process fine pore structures up to 20% to 50% porosity can

be achieved by sintering. A large variety of materials have been used for this

process including titanium, super alloys, bronze and steels (Srivastava and

Sahoo 2007).

Zhao et al (2005) reported that the raw materials for manufacturing

foam are metal and carbonate in powder form, this leads to the formation of

carbon monoxide gas up on melting of the components. The particle size of

carbonate powder needs to be selected according to the intended cell size of

the final foam.

1.3.4.2(c) Foaming of slurries

Metallic foams can be produced by preparing slurry of metal powder

mixed with a foaming agent. The slurry is poured into a mold after mixing

and dried at elevated temperatures. This slurry becomes more viscous and

starts to foam as gas begins to evolve. If proper stabilizing measures are

taken, the slurry can be dried completely obtaining metal foam (Banhart

and Baumeister 1998). Figure 1.19 shows the steps involved in foaming of

slurry.

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Figure 1.19 Foaming of slurries

Baumeister et al (1994) reported that the method comprises foaming

of a slurry consisting of fine aluminium powder and an organic vehicle.

Foaming of slurry is affected by whipping or a chemical reaction. The foamed

slurry was cured for 2 hours at 100° C to increase the mechanical strength of

the foam.

1.4 NEED FOR THIS RESEARCH

Metal foams considered as revolutionary material because of its

excellent physical and mechanical properties. Due to the consumer demand

and stiff competition present day industries are forced to produce low density

products with cost-effective properties. Porous materials satisfy the above

need of the industries. In this literature review application in various fields

and evolution of metal foams were discussed. Production methods of metal

foams were also discussed in detail that includes different types of liquid

metallurgy route and powder metallurgy route.

From the literature review it is very clearly seen that, no attempt was

made for production of porous gunmetal castings in any of the method. Due to

this factor an attempt was made to develop porous gunmetal casting by melt

route method employing space holder technique. Inorganic cores were used

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for the development of pores in the gunmetal castings. The technique stands

as an economical method.

Stainless steel and aluminium find application in various fields

including medical and engineering. Hence the melt route method by using

space holder technique sees to be promising and economical due to its

development procedure. From the literature review it is clearly seen that no

attempt was made to develop porous stainless steel and porous aluminium by

this technique earlier. In this research an attempt was made to develop both

porous stainless steel and porous aluminium by space holder technique using

inorganic cores to produce the pores on the castings. The layout for

production of porous castings is given in Figure 1.20.

Figure 1.20 Layout for production of porous castings