Development of Closed Cell Metallic Foam Using Casting Techniques

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Abstract

O’Neill, Adrian Thomas. Development of Closed Cell Metallic Foam Using Casting Techniques. (Under the Direction of Dr. Afsaneh Rabiei.)

The research sited in this paper involves the development of a new metal foam composite

material using casting techniques. This work included the design of the material and the

development of a process to produce the metal foam. The materials used to produce the

foam consisted of low carbon steel hollow spheres and an aluminum alloy. The foam is

comprised of steel hollow spheres packed into a random dense arrangement, with the

interstitial space between spheres infiltrated with a casting aluminum alloy. Using

prefabricated hollow spheres assures a uniform pore size and cell wall thickness. Casting a

metal into the interstitial space provides a solid media to add structural support to the foam.

The goal of this research has been to develop metal foam that demonstrates improvements in

product uniformity and mechanical properties over the currently available foams. To

accomplish this goal, the study included the identification of the various technologies used to

manufacture metal foams, the assessment of the improvements needed to augment the quality

of foamed metals, and the design of a new product and processing technique that

substantiates these goals. The experimental equipment was designed and procured, while the

raw materials were obtained. Then the hollow sphere foam samples were successfully

produced. Using these samples a series of characterization studies was done to qualify and

quantify the results. These findings were then compared to presently published data to gauge

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The hollow sphere metal foam developed in this study displayed significant improvements in

the measures of compressive strength and energy absorption capacity, all the while

maintaining the characteristic properties of cellular metals. The improvements were

measured against the next best existing technology. The newly developed foam averaged 67

MPa over a region of 10 – 50% strain, with densification beginning at approximately 50%

strain. The value for energy absorption is 30 MJ/m3 at 50% strain. This foam also has a

strength to density ratio on level with the best reported results to date. The combination of

these properties gives opportunity for use in previously unidentified applications, such as an

energy absorption media for buildings subject to seismic motion. This foam can also be

designed in such applications as automobile crumple zones, as structural members in air and

space craft, and in biomedical prosthesis.

Several areas for improvement have been identified for this technology. The bonding

strength between sphere and matrix needs improvement, and different material choices and

processing changes have been identified in this research to achieve these improvements. The

packing density of the spheres can be improved, and a new method of vibrating the sphere

arrangement prior to molding may increase the packing density. The porosity of the

aluminum matrix can be reduced, and the design of the casting mold and processing

conditions can be modified to reduce undesirable porosity. Additional testing methods have

been identified to further characterize the foam and reveal insights for further improvement.

The iterative process of sampling, characterization, and analysis will continue to improve this

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DEVELOPMENT OF CLOSED CELL METALLIC FOAM USING CASTING TECHNIQUES

by

ADRIAN THOMAS O’NEILL

A Thesis Submitted To the Graduate Faculty Of North Carolina State University

In Partial Fulfillment Of The Requirements for the Degree Of

Master of Science

MECHANICAL & AEROSPACE ENGINEERING

Raleigh

Fall 2004

APPROVED BY:

_____________________________________ Dr. Afsaneh Rabiei – Chair of Advisory Committee

_____________________________________ Dr. Jeffrey W. Eischen – Member of Advisory Committee

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Dedication

This publication is dedicated to Margaret, my mother, and Elana, my sister, whose

unwavering support has given me the strength and guidance to persevere in achieving my

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Personal Biography

Adrian T. O’Neill received his Bachelors of Mechanical Engineering degree in 1998 from the

Georgia Institute of Technology, receiving highest honors. His major area of study was

Mechanical Engineering with a specialization in fluid and thermal systems.

He went on to work at the Goodyear Tire & Rubber Company in Akron, Ohio. At Goodyear,

after doing a hands-on introduction to tire manufacturing in the race tire division, he was an

associate in the passenger tire development division. There he utilized CAD software and

performed finite element analysis in designing and developing tire tread patterns for new

lines of tires. He was awarded ten patents as the co-inventor for novel tire tread designs.

In 2000, he was hired into the consulting firm of Plastic Technologies, Inc. in Toledo, Ohio.

As an associate engineer, he designed, developed and tested packaging for the food industry,

made from polyethylene terephthalate (PET) and polypropylene.

In the fall of 2003, he enrolled as a graduate student at North Carolina State University. He

is currently working towards a master’s degree in Mechanical Engineering with the

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Acknowledgements

The research and findings included in this thesis were accomplished with the help and

support of many persons. I would like to acknowledge and thank these persons for the

generous gift of their time and knowledge.

My advisor on this project, Dr. Afsaneh Rabiei, was a consistent source of help and

encouragement. My advisory committee, Dr. Afsaneh Rabiei, Dr Jeffrey Eischen, and Dr.

William Roberts have generously given their time to review my work.

Laboratory testing of samples was accomplished with the assistance of Dr. Sami Rizkalla and

Lee Nelson in Civil Engineering, Dr. Harvey West in Industrial Engineering, and Dr. Dale

Batchelor and Chuck Mooney in the Analytical Instrumentation Facility.

Experimental tools were fabricated by Skip Richardson and Mike Breedlove in the

Mechanical & Aerospace Engineering department machine shop.

My fellow graduate students and members of my group, Brian Neville, Wassim Azzi, and

Elizabeth Baldwin created a co-operative and supportive environment in which we

accomplished whatever the moment required.

My gratitude also extends to the Graduate School of Engineering for the fellowship award

that enabled me to work on this project, and to the National Science Foundation for funding

this research.

I would be remiss not to acknowledge and thank the people of the state of North Carolina for

providing and supporting this great institution in which I have been able to continue my

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List Of Tables

Table 1. Physical And Mechanical Data For Georgia Tech HSF With λ/D = 2, λ/D = 5,

And λ/D = 10. 17

Table 2. Mechanical and Physical properties of Fraunhofer HSF shown in Figure 8 (c). 21

Table 3. Physical And Mechanical Properties Of Magnesium Syntactic Foam. 24

Table 4. Physical And Mechanical Properties Of Alporas Foam. 30

Table 5. Physical And Mechanical Characteristics Of Cymat SAF. 33

Table 6. Typical SEM Settings Used In Imaging HSF Samples. 44

Table 7. Aluminum Alloy Compositions As Delivered By Trialco, Inc. 47

Table 8. Constants Used To Determine HSF Relative Density. 51

Table 9. Results Of Packing Density Experiment. 54

Table 10. Physical And Mechanical Characteristics Of NCSU HSF. 57

Table 11. Qualitative Elemental Analysis On HSF Features. 76

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List Of Figures

Figure 1. (A) ERG Duocel Open Cell Foam Used In A Heat Exchanger, And (B)

Alulight Closed Cell Foam Used As Structural Support In A Hollow Beam. 3

Figure 2. Representative Stress Strain Curve for Metal Foams. 4

Figure 3. Cymat Corporation (A) Bumper Design Incorporating Crashboxes, (B)

Compressed Crashbox, And (C) Additional Crashbox Designs. 7

Figure 4. Liquid Metallurgy Processing Techniques Of Cellular Metals. 12

Figure 5. Non-Carburized Fe – 12 Cr Foam Before And After 70% Compressive Deformation. 14

Figure 6. Compressed Stainless Steel HSF Samples Of Varying Sizes, Corresponding

To λ/D Ratios Of 2, 5, And 10. 15

Figure 7. Stress – Strain Behavior Of The Samples Shown In Figure 6. 16

Figure 8. Fraunhofer HSF Samples Made From (A) Various Metals, (B) 316L Stainless Steel, And (C) 316L Stainless Steel. 18

Figure 9. Fraunhofer Powder Metallurgy Production Process For Hollow Spheres. 19

Figure 10. Fraunhofer - Cross Sectional View Of Two Spheres In Contact. 20

Figure 11. Stress Strain Curve Of Fraunhofer 316L Stainless Steel HSF Shown In Figure 8 (c). 21

Figure 12. Infiltration Casting Process Used To Produce Magnesium HSF. 23

Figure 13. Magnesium Matrix / Aluminum Oxide Sphere HSF Samples. 23

Figure 14. (A) Stress Strain Behavior Of Magnesium Syntactic Foam, And (B)

Photograph Showing Shear Band During Compression Deformation. 24

Figure 15. Defects In Magnesium Syntactic Foam (A) Inner Wall Of Hollow Sphere Penetrated By Magnesium Droplet, And (B) Two Hollow Spheres With Incomplete Infiltration Of Magnesium In Interstitial Space And Non-Uniform Wall Thickness.

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Figure 17. Alporas Foam (A) With Mean Cell Size Of 3.0 mm, And A Cell Size

Distribution (B) Ranging From 1-7 mm. 28

Figure 18. Alporas Foam (A) With Mean Cell Size Of 4.5 mm, And A Cell Size

Distribution (B) Ranging From 1-13 mm. 28

Figure 19. Apparent Density Distribution Of From The Cross Section Of (3) Alporas Foam Samples. The Apparent Density Range Is 0.18–0.24 g/cm3. 29

Figure 20. Behavior Of The Two Different Alporas Foams In Compression Testing. The Strain Rate Is 1 x 10-3 s-1, And The Mean Cell Sizes Are (a) 3 mm And (b) 4.5

mm. 29

Figure 21. Cymat SAF Production Process: Direct Foaming Of Al-Melt By Gas

Injection. 31

Figure 22. 90x90x50mm Cymat Foam, 10% RD. 32

Figure 23. Stress Strain Curves For Cymat SAF, Relative Density Ranges Between 15.5% - 22.4% And Strain Rates Range Between 10-3 – 10+1. 32

Figure 24. HSF Mold In Various Stages Of Assembly (A) Runner With Gate, (B) Adding The Filter And Side Walls, And (C) Fully Assembled. 36

Figure 25. HSF Mold Shown In (A) Side View And (B) Top View. 36

Figure 26. Molding Tools Used In Casting, Including Tongs, Crucible, And Quartz Stirring Rod. 38

Figure 27. Buehler Isomet 4000 linear precision saw. 39

Figure 28. Buehler Automet 2 Power Head Grinding And Polishing Systems. 40

Figure 29. Buehler Ultramet 2002 Sonic Cleaner. 41

Figure 30. 220-Kip MTS-810 Closed-Loop Universal Testing Machine. 42

Figure 31. Unimet Unitron 9279 Optical Microscope. 43

Figure 32. Fraunhofer Hollow Spheres (a) Several, (c) Cross Section Of A Single Hollow Sphere, And (c) Micrograph Showing Wall Porosity. 45

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Figure 34. (A) The Mold In A State Of Preassembly, Showing The Bottom Channel, Ceramic Filter, Bottom Screen, And Two Walls. (B) Mold Fully Assembled For Trial With Solid Iron Spheres. (C) Mold And Crucible In Furnace. (D) Fully Heated Mold And Crucible In Furnace. 49

Figure 35. First Sample Produced Using Hollow Spheres. 50

Figure 36. Photograph Of Packing Density Experimental Setup. 54

Figure 37. Stress – Strain Data For HSF Compression Testing. 56

Figure 38. Absorption Energy Diagram Comparing HSF To Solid 356 Al Alloy. 57

Figure 39. Compression Testing Of Sample No. 2 In The First 11 Minutes, Loading Up To 17,500 Lbs And 26.2% Strain. 59

Figure 40. Photos Of Compressed Sample No. 1 Showing A) Densification (60.7% @ 56,300 Lbs), And B) Remaining Debris After Testing (17% By Weight). 60

Figure 41. HSF Samples Mounted In Epoxy Used For SEM Imaging. 61

Figure 42. SEM Image Showing Two Spheres In Direct Contact Within The Matrix. 63

Figure 43. SEM Image Showing Four Spheres In Close Proximity And Extent Of Matrix Penetration. 64

Figure 44. SEM Image Showing Four Spheres In Close Proximity. Material In Top Sphere Is Epoxy From Mount. 65

Figure 45. SEM Image Showing The Cross Section Of A Hollow Sphere Wall And The Interface Layer Between Sphere And Aluminum Matrix. 66

Figure 46. SEM Image Showing Two Spheres In Close Proximity With Aluminum Matrix Filled In Between. 67

Figure 47. SEM Image Of Interface Between Hollow Sphere And Aluminum Matrix. 68

Figure 48. SEM Image Taken With Specimen Current Set High To Illuminate Void Space In The Sample. 69

Figure 49. SEM Image Showing The Interface Between Sphere And Matrix, Showing That Cracking May Be Surface Effect. 70

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Figure 51. SEM X-Ray Mapping Indicating The Compositional Analysis Of Interface Between The Steel Sphere (Right), An Intermediate Layer (Middle), And A

Precipitate In The Aluminum Matrix (Left). 72

Figure 52. SEM Image Showing Aluminum Matrix And Precipitates Contained

Within. 73

Figure 53. SEM X-Ray Mapping Indicating The Compositional Analysis Of The

Aluminum Matrix. 74

Figure 54. SEM Image Showing Locations For Quantitative Compositional Analysis. 75

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List Of Symbols And Abbreviations

Symbol or Abbreviation Term

AIF Analytical Instrumentation Facility

BSE Back Scatter Electron

CFL Construction Facilities Laboratory

CVD Chemical Vapor Deposition

EDM Electro-discharge Machining

EDX Energy Dispersive X-ray Spectroscopy

EPS Expanded Polystyrene Spheres

HSF Hollow Sphere Foam

MMC Metal Matrix Composite

PM Powder Metallurgy

PVD Physical Vapor Deposition

SAF Stabilized Aluminum Foam

SEM Scanning Electron Microscope

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Introduction

The first metal foam was invented in 1943, by Benjamin Sosnick of San Francisco California

[1]. He created a “sponge metal” using mercury as a foaming agent in molten aluminum. It

was not until the 1990’s, however, that an intensified research effort began. This effort was

successful in developing new technologies that have brought several metal foams to the

marketplace. The list of metals and alloys that have been foamed is extensive. Among the

most popular are aluminum, iron, and titanium. Research has been done to develop metal

foams made from carbon, copper, lead, tin, and zinc. Additionally, foam has been

developed using a base of carbon foam with a deposit of hafnium, niobium, rhenium,

tantalum, tungsten, or zirconium over the carbon foam base [2]. Foams have been developed

using platinum and silver [3], silicon carbide [4], nickel [5], and molybdenum [6]. The

research into the manufacturing of these cellular metals has developed various techniques to

include: metal casting, powder metallurgy (PM), electro deposition, chemical vapor

deposition (CVD) and physical vapor deposition (PVD) [7]. Casting and PM are common

techniques used to manufacture larger quantities of foams made from steel, titanium and

aluminum, whereas electro deposition, CVD and PVD are used to produce more exotic

foams such as those using refractory metals.

The development of metal foams introduced improved properties when compared to

non-metal foams and solid non-metals. Compared to non-non-metal foams, non-metal foams offer higher

stiffness, better strength to weight ratios, increased impact energy absorption, and a greater

tolerance to high temperatures and adverse environmental conditions. In comparison to solid

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the size, shape, and volume fraction of cells, mechanical properties can be engineered to

meet the demands of a wide range of applications [8].

The characteristic properties that define a metal foam include its cellular structure and

relative density. Metal foams are either open cell, closed cell, or a combination of the two

[9]. Open cell foam can be thought of as a network of interconnected solid struts. Open cell

foam will allow fluid media to pass through it. Closed cell foam is made up of a network of

adjacent sealed pores, all sharing walls with each other. A fluid media can not pass through

closed cell foam. A combination of open cell /closed cell composition is technologically

possible. This is achieved in several differing ways. An example is a closed cell foam

whose cell walls have been partially fractured. Another example is a stacked arrangement of

hollow spheres sintered together. The pore space inside the hollow spheres represents the

closed cell nature, but the open interstitial space between the spheres constitutes an open cell

nature. Figure 1 shows an example of ERG Duocel [4] open cell foam used in a heat

exchanger (a), and Alulight [10] closed cell foam used as structural reinforcement filling a

hollow beam (b). The type of cellular structure in a metal foam will give the foam different

inherent characteristics.

The physical properties of cellular structure and relative density of the manufactured material

determine whether it is classified as a foamed metal. The most important parameter

characterizing a foamed material is its relative density. This is defined as the quantity ρ*/ρs.

ρ* is the density of the cellular solid itself, and ρs is the density of the material from which it

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Figure 1. (A) ERG Duocel Open Cell Foam Used In A Heat Exchanger, And (B) Alulight Closed Cell Foam Used As Structural Support In A Hollow Beam.

density of 0.35g/cm3 (ρmagnesium = 1.74g/cm3) is 0.20, or 20%. As the cell walls thicken and

the pore size decreases, the relative density increases. The term “cellular structure” refers to

a media with a relative density less than 30%. Otherwise, it is considered a solid structure

containing isolated pores, or porous media. In terms of cellular metals, the term “foam”

technically refers to a liquid suspension of gas within a molten metal, but is commonly used

to discuss cellular metals with relative densities less than 30%.

Figure 2 shows a stress – strain curve of an elastic-plastic foam under compressive loading

[11]. This graph illustrates some of the key defining mechanical properties. The Young’s

modulus is the ratio of stress to strain in the linear elastic region. In the elastic region, the

metal foam displays cell wall bending. The plateau stress level is defined as the average

yield stress over the course of plastic deformation before densification. In most metal foams,

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Figure 2. Representative Stress Strain Curve for Metal Foams.

region, the foam displays cell collapse until the pore space is closed. In the densification

region, the foam acts like a solid piece of metal. The densification strain is the strain at the

point where the stress – strain curve approaches near vertical.

Another mechanical property of metal foam that can be derived from the stress-strain curve is

the impact energy absorption capacity [11]. This is defined as the area under the curve up to

a certain strain. The defining value often quoted for metal foams is the energy absorbed at

50% strain. Any foam’s ability to absorb impact energy makes it useful in packaging to

protect breakable items. Metal foams find use in protecting objects subject to higher forces,

such as automobiles in collisions. Metal foams can deform plastically over a large region of

strain all the while maintaining a low stress level. The slowed deceleration keeps the peak

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At this time, the mechanical properties of metal foams are in fact unpredictable. This is due

to imperfect production techniques that create an inconsistent material. The cellular structure

is often heterogeneous across the sample, creating varying mechanical properties. This

makes it difficult to engineer products using metal foam, as the extent and location of failure

can not be adequately predicted. The cellular structure in closed cell metal foams is typically

disordered, due in part to three main imperfections [12]. These phenomena are wavy

distortions of the cell walls, cell wall thickness variation, and non-uniform shape and size of

the cells.

As opposed to a uniform cell shape, such as a square or round cell, wavy distortions in cell

walls produce irregularly shaped cells. This phenomena leads to reduced stiffness of that

cell, as an irregular cell wall shape will not hold the same load. If the cellular material

comprises several irregularly shaped cell walls distributed among the foam, it is impossible

to predict what the overall strength of the foam is and where it will fail under loading.

Similarly, wall thickness variation and cell size variation contribute to the non-uniformity of

cellular structure, and additionally contribute to the anisotropic physical and mechanical

properties making it difficult to predict the foam’s performance characteristics.

The current cost of metal foams prohibits its use in many potential applications. Although

production lines capable of producing large volumes of foam exist, the quality of this foam is

not up to the standards required by most applications. Due to the high cost of production

processes and some of the exotic metals used, foams are limited to niche applications or

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increased recognition of new uses and the resulting demand, incentives will surface for

continued research and development among industrial and academic institutions.

This thesis outlines the efforts toward the development of hollow sphere metal foam using

liquid metal casting techniques, conducted by the Advanced Materials Group within the

North Carolina State University Mechanical and Aerospace Engineering department. Hollow

sphere foam (hereafter referred to as HSF) is a closed cell foam, produced using casting

techniques. Therefore, the focus of the following literature review will be on closed cell

technology produced using liquid metallurgy.

Potential Applications

The potential applications for cellular metals have grown rapidly. There are several

identified uses in industry in the fields of automotive design, marine and rail transportation,

aerospace engineering, biomedical engineering, architectural and structural design, machine

construction, and many additional functional applications [13]. Some of these will be briefly

discussed here.

Automotive Design

Several applications have been identified within the arena of automotive design. This area

takes advantage of three major attributes of metal foams, including energy absorption

capacity, high strength coupled with low weight, and good damping / sound absorption

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Cymat [14] has invested considerable research efforts in identifying uses for their aluminum

foams in automotive design, including bumper crash boxes, shown in Figure 3. This replaces

traditional steel crumble zone members with lighter weight, higher impact energy absorption

efficient crash boxes. This design reduces damage to the car with more energy absorbed

locally, and absorbs energy in off axis impacts better than standard crumple members. Metal

foam can be used to fill hollow sheet metal sections to increase stiffness, dampen road

vibration, and buffer road noise. Such sections include safety rails in doors, and A and B

pillars. A metal foam sandwich panel could be used for an engine hood. This could increase

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safety in the event of a pedestrian collision, as well as increasing heat dissipation and

absorbing engine noise.

Sound Absorption

Metal foams can absorb sound and dampen vibration [13]. A sound wave incident to a foam

structure is partially reflected but also partially absorbed. The sound which enters into the

structure is attenuated inside the foam, especially if the pores are interconnected by small

channels. The sound wave presses air through these channels numerous times per second.

As the air flows through the channels, friction between air and cell walls dissipate energy.

Metal foams can reach quite high absorption levels, up to 99% for certain frequencies (1-5

kHz). The loss factor of foams is at least 10 times higher than solid metal, so vibration is

damped out more efficiently, i.e. converted to heat, therefore reducing vibration damage and

emission of sound waves.

Alporas foam has been used as sound barriers on the underside of raised expressways in

Japan [15]. It is initially a closed cell foam, but is rolled to a strain of 10%, just enough

deformation to crack some of the cell walls to create pathways for sound waves to travel. In

addition to noise attenuation, the material is fire resistant, weather resistant, and is non-toxic

if exposed to flames. Some also find it to be aesthetically appealing.

Biomedical Engineering Design

Titanium or cobalt-chromium alloys are used in biomedical prosthesis design due to good

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incorporate a porous outer surface to encourage bone in-growth and bonding between the

bone and medical implant. Porous structures, such as metal foams, can be used in place of

metals incorporating a porous outside layer.

One opportunity for the use of metal foam to improve on the functionality of solid metal

implants lies in the ability to adjust the foam’s mechanical properties. An example of a

replacement hip joint illustrates this.

The dissolving and assimilation of the surface of the femur bone in contact with a femoral

hip replacement component contributes to premature wear and reduces the overall life of the

prosthesis. It has been suggested that this may be due to the artificially induced state of

stress and strain in the femoral cortex. When a titanium femoral stem is inserted into the

bone, it deprives the bone of its customary load carrying duty as a result of the implant’s

increased stiffness. Bone customarily adapts to establish a stable mechanical environment.

In this scenario, bone is resorbed because it does not contribute to the stiffness of the joint

structure. Therefore, to maintain bone mass, the replacement prosthesis should mimic the

mechanical characteristics of the bone it is replacing.

The mechanical properties of metal foams can be adjusted by varying the physical

characteristics of cell size, shape, and distribution. It is feasible to engineer metal foam,

using a biocompatible metal, to possess physical and mechanical properties matching that of

bone. This would resolve the bone resorption issue associated with stiffer solid metal

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Research Objectives

The goal of this research is to produce high-strength, light-weight metal foam. Specifically,

it will be a stacked array of hollow spheres with metal infiltrating all the interstitial space.

This metal foam material is to possess all the positive attributes realized in current metal

foam technology, such as high specific stiffness, low relative density, and a high impact

energy absorption capacity. This material will also be designed to improve on the recognized

deficiencies of current metal foam. The fundamental deficiencies with current metal foam,

as mentioned previously, are wavy distortions of the cell walls, cell wall thickness variation,

and non-uniform shape and size of the cells. This heterogeneity in physical properties leads

to non-uniform, anisotropic material properties, which are problematic when attempting to

design metal foams into new applications.

To realize these goals, the theoretical design of the metal foam will be completed. This

includes specifying the hollow sphere attributes, including metal type, sphere size and wall

thickness. Dr. Afsaneh Rabiei, the project advisor and PI, worked in conjunction with the

Fraunhofer Institute to design and produce steel hollow spheres. The method for producing

the hollow spheres is outlined in the section on Fraunhofer HSF.

The next step is to design the production process for samples. A sample size will be chosen,

and a permanent mold casting system will be designed. It will be designed for repeated use,

and will also be designed to be modular so that changes to the mold can be made with the

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It will then be necessary to procure all the equipment needed for making samples, including a

high temperature furnace, casting tools, and safety equipment. Samples will then be made,

first with less expensive solid steel spheres. When the processing parameters are finalized,

the hollow spheres will be used.

The next objective is to test the samples made and qualify / quantify the foam’s performance.

Compression testing will indicate the specific strength and energy absorption capability.

SEM imaging will indicate the degree of bonding of the spheres to the matrix, and verify the

composition of the various phases in the spheres, matrix, and the interface between the two.

The performance of this foam will be compared to currently published data to gage the

relative success of progress made.

Lastly, the knowledge gained from the literature survey and experimental work will be

summarized. This information will be used to plan out the next steps in this research to

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Literature Review

Figure 4. Liquid Metallurgy Processing Techniques Of Cellular Metals.

Liquid Metallurgy Techniques

Figure 4 shows various liquid metallurgy processing techniques [17]. Casting metal foam

occurs when a liquid metal is poured into and around a space-holder material. Casting

around hollow spheres or stirring in space-holding particles into liquid metal are examples

where the porous substrate remains embedded in the cellular solid. This technique is used to

create closed cell metal foams. When liquid metal is cast around a removable substrate, this

technique creates open cell structures. An example is investment casting, where the substrate

material is burned out of the metal to create the interconnected open cellular structure.

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gas injection, such as blowing in air bubbles, or by stirring in particles which dissociate into

gas bubbles.

Examples of each of these techniques for creating closed cell metal foams will be described.

This includes the current hollow sphere foam technology developed by the Georgia Institute

of Technology, the Fraunhofer Institute, and the University of Erlangen, as well as Cymat

aluminum foam created by direct gas injection, and Alporas aluminum foam created by using

the dissociating blowing agent TiH2.

Georgia Tech HSF

The Georgia Institute of Technology has generated hollow sphere structures made from

coaxially sprayed slurries [18,19,20]. This foam is a combination open cell / closed cell

structure, having a certain volume fraction of pore space inside the closed hollow spheres,

but it also has an open lattice structure with space in the interstitial network of spheres. In

this method, spheres are produced using a coaxial nozzle, and sprayed into a predefined

molded shape. The network of spheres is then a random distribution without significant

order to the stacking configuration.

Processing

Hollow spheres can be made through the use of a coaxial nozzle. A slurry of metal powder,

solvent, and a polymer binder is sprayed through the outer jet (concentric orifice) of a coaxial

nozzle, and a gas is blown through the center of the nozzle. By controlling the flow rate of

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form of a hollow cylinder, and due to hydrostatic forces and surface tension, the cylinder

walls close on themselves creating hollow spheres. As the solvent portion of the slurry dries,

the spheres harden resulting in dry polymer-bound powder spheres. This process can

produce 3,000 to 15,000 spheres per minute or 2 to 7 kg/hr from a single nozzle. These

spheres are later heat treated to pyrolyze the polymeric binder material and are then sintered

to form a solid metallic shell. The hollow spheres can then be sintered together to form a

bulk foam structure. The sizes of the spheres can be varied to range from 1-6 mm and the

wall thickness can be varied from 40 to 200 µm. Figure 5 shows a before and after picture of

a foam sample that has been compressed to 70% strain.

Figure 6 shows bulk metal foams that have been compression tested. The λ/D ratio is the

ratio of specimen width to outer sphere diameter. This is varied to determine the necessary

λ/D ratio so that the data obtained from the test sample is representative of a larger bulk

metal foam.

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Figure 6. Compressed Stainless Steel HSF Samples Of Varying Sizes, Corresponding To λ/D Ratios Of 2, 5, And 10.

Characteristics

The Georgia Tech hollow spheres were produced using an 88.2 wt.% Fe2O3 and 11.8 wt.%

Cr2O3 powder mix. The slurry also contained an acetone dispersant and poly(methyl

methacrylate) (PMMA) binder. After thermal processing in a hydrogen atmosphere, the

metal composition is nominally that of a 405 ferritic stainless steel (Fe-12Cr). The spheres

produced have an outside diameter of ~2 mm, with a wall thickness of ~0.1 mm. The wall

thickness varied from sphere to sphere by as much as 50% due to gravity and inertial effects

in the dynamic processing of spheres. The reduced spheres are vibrated to increase the

packing density, and are then sintered at 1350°F for 48 hours to promote bonding through

inter-diffusion between of the metal spheres. The resulting foam has a density of 1.4 g/cm3,

corresponding to a relative density ρ∗/ρs of 0.16.

Compression testing was conducted at room temperature using a strain rate of 10-3 s-1. Figure

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Figure 7. Stress – Strain Behavior Of The Samples Shown In Figure 6.

Table 1 summarizes the physical and mechanical properties of the Georgia Tech samples.

For the smaller samples, the failure mechanism was fracture of the sphere necks, or the

bonding location between spheres, leading to sphere separation on the edges of the bulk

sample. Larger samples were cut using electro-discharge machining (EDM) so that the cross

section could be viewed in compression testing. In these larger samples, a non-uniform

deformation dominated compression behavior. As the λ/D ratio increases, the results become

more repeatable, showing that a sample with a λ/D ratio of 10 is statistically representative of

a large bulk foam sample. The stress / strain curves for the λ/D ratio of 10 were digitally

reproduced in MS Excel and the area under the curve was estimated using the Riemann Sum

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Table 1. Physical And Mechanical Data For Georgia Tech HSF With λ/D = 2, λ/D = 5, And λ/D = 10.

GA Tech

λ/D = 2

GA Tech

λ/D = 5

GA Tech

λ/D = 10

Sphere Outside Diameter (mm) 2 2 2

Sphere Wall Thickness (mm) 0.1 0.1 0.1

Density (g/cm3) 1.4 1.4 1.4

Relative Density (%) 16 16 16

Plateau Stress (MPa) 3-15 3-6 5-7

Densification Strain (%) 65 65 65

Energy Absorption @ 50% Strain (MJ/m3) not reported, estimated 2.1 for λ/D = 10

Strength / Density Ratio ~6 ~3 ~4

Fraunhofer Hollow Sphere Foam

The Fraunhofer Institute for Manufacturing and Advanced Materials has developed hollow

sphere foams using various metals, including titanium, nickel and stainless steel [21,22,23].

Figure 8 shows foams using hollow spheres of these varied materials. The process for

manufacturing the hollow spheres is shown in Figure 9. Polystyrene granules are expanded

to create expanded polystyrene spheres (EPS). The advantages of using EPS include low

cost, dimensional uniformity and a clean removal during pyrolyzation. The EPS are floated

in a fluidized bed, where they are coated with a slurry of metal powder and binder liquid.

This method provides for an even coating on the EPS and instant drying of the binder liquid.

The coated spheres are then heated to pyrolyze the EPS and the binder liquid. The spheres

are then sintered to bind the metal powder particles together and leave a hollow hardened

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Figure 9. Fraunhofer Powder Metallurgy Production Process For Hollow Spheres.

Processing

The green spheres can be compacted into a mold of predefined shape and sintered as a bulk

material to create hollow sphere foam. One advantage of using green spheres to create the

bulk shape is the increased contact between spheres. Since the spheres have not been heat

treated at this point, the spheres are still formable, but they will not collapse due to the EPS

inside. If the spheres were sintered before they were used to create a HSF, there would be

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Figure 10. Fraunhofer - Cross Sectional View Of Two Spheres In Contact.

spheres in contact. Note the extended surface contact and good bonding. To create the foam

sample shown in Figure 8 (c), the bulk material was sintered at 1250°C for one hour in a

hydrogen atmosphere.

Characteristics

Compression testing was done with a cross head speed of 10mm/min. Figure 8 (c) shows the

sample before and after compression testing. The stress – strain curve in Figure 11 shows the

resulting compression results, and Table 2 summarizes the physical and mechanical

properties. The outside diameters of individual spheres range from 2-3 mm. The energy

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Figure 11. Stress Strain Curve Of Fraunhofer 316L Stainless Steel HSF Shown In Figure 8 (c).

Fabrication and Properties of Syntactic Magnesium Foams

Scientists in the Department of Material Science at the University of Erlangen in Germany

have developed a magnesium syntactic foam utilizing hollow spheres [24]. Syntactic foams

are those which use a placeholder material, different from the matrix material, to create the

pore space. This creates a three phase composite, including the matrix material, the cell

Table 2. Mechanical and Physical properties of Fraunhofer HSF shown in Figure 8 (c).

Fraunhofer 316L SS HSF

Sphere Outside Diameter (mm) 2-3

Sphere Wall Thickness (mm) 0.250

Sample Dimensions (Dia. x H) (mm) 24.7 x 23.4

Density (g/cm3) 1.43

Relative Density (%) 21

Plateau Stress (MPa) 20-40

Densification Strain (%) 60

Energy Absorption @ 50% Strain (MJ/m3) not reported, estimated to be 10

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material, and the gas inside the cells. In this technology, thin walled hollow alumina spheres

are embedded in a magnesium matrix. Liquid magnesium infiltrates an alumina sphere array

using a pressure casting technique.

Processing

The alumina spheres were processed using a patented powder slurry sintering process [25].

Four variables of spheres were used with dimensions as follows:

• 2.8 mm outside diameter, 133 µm wall thickness, average sphere density = 0.81 g/cm3

The standard deviation for the sphere diameter was 7%, and the standard deviation of wall

thickness dimension was 40-60%. A vacuum assisted sedimentation process was used to

remove defective spheres. Four magnesium matrix alloys were used, specifically cp-Mg,

AM20, AM50, and AZ91.

A schematic of the infiltration casting technique is shown in Figure 12. The hollow alumina

spheres are vibrated in a steel mold to achieve dense packing. Liquid magnesium is cast into

the interstitial space using pressurized argon gas. The melt temperature is held at 700°C, the

die temperature at 400°C, the vacuum held in the mold is 0.1 bar, the infiltration pressure is

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Figure 12. Infiltration Casting Process Used To Produce Magnesium HSF.

Characteristics

The composite foam density varied between 1.0 and 1.4 g/cm3. Assuming no porosity was

present in the magnesium matrix material, the volume fraction of the spheres is assumed to

be 63% based on a rule of mixtures approach and assuming random packing of spheres. The

porosity of the syntactic foams ranged from 42 to 52%, knowing the inside diameter of the

hollow spheres. A picture of the finished product is shown in Figure 13.

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Figure 14. (A) Stress Strain Behavior Of Magnesium Syntactic Foam, And (B) Photograph Showing Shear Band During Compression Deformation.

Compression testing was done using an Instron machine with a strain rate of 5.5 x 10-3 s-1.

Figure 14 shows the stress strain behavior and an optical micrograph demonstrating the

tendency to form shear bands while deforming. Table 3 summarizes the physical and

mechanical properties of a AM20 magnesium syntactic foam sample.

Table 3. Physical And Mechanical Properties Of Magnesium Syntactic Foam.

Magnesium Syntactic AM20 Al2O3 HSF Sphere Outside Diameter (mm) 3.7

Sphere Wall Thickness (mm) 0.150 Sample Dimensions (L x W x H) (mm) 20 x 20 x 30

Density (g/cm3) 1.16

Relative Density (%) 36.4

Plateau Stress (MPa) 46

Energy Absorption @ 50% Strain (MJ/m3) not reported, estimated to be 23

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Advantages/Disadvantages

The foam quality is sensitive to the infiltration process conditions. If the infiltration pressure

is too high, the magnesium can penetrate the alumina spheres, filling the pore space, and

increasing the foam density. If the infiltration pressure is too low, the interstitial space

between spheres may be incompletely filled. The infiltration pressure necessary to raise the

column of magnesium, i.e. counter the metalostatic pressure needed to completely fill the

part, may be high enough to infiltrate spheres. Figure 15 shows an example of both

phenomena.

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Alporas Foam

Alporas is an ultra-light aluminum closed cell metallic foam produced by the Shinko Wire

company in Japan [26].

Production Process

Alporas is made through a batch casting process via stabilizing gas bubbles in an aluminum

melt. The process is shown in Figure 16. Pure aluminum is melted at 680°C in a large

container with 1.5 wt% calcium mixed in. The calcium oxidizes in the melt, creating a

dispersed suspension of CaO and Al4Ca particles. These particles increase the viscosity of

the melt, thereby serving to stabilize the gas bubbles in the next step. Next, the melt is

poured into a mold, with 1.6 wt% TiH2 particles added to foam the aluminum. At 680°C, the

hydrogen dissociates from the titanium creating hydrogen gas bubbles in the aluminum, thus

yielding a porous structure. Once the melt is foamed and expanded to the extents of the

mold, it is cooled. The foamed block is released from the mold and sliced into usable size

pieces.

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Characteristics

Illustrations of two different pore size Alporas foam samples are shown in Figure 17 and

Figure 18. The first has a generally smaller pore size versus the other, which can be

attributed to the extent of dissociation of hydrogen gas from the TiH2 particles. The

volumetric yield of gaseous hydrogen depends on the temperature of the melt and the length

of time spent in the expansion phase. Figure 19 shows the density distribution through a

cross section of a sample. The density is higher at the surfaces than in the center of the

block. The sample solidifies from the outside inward, therefore the interior is allowed to

foam longer than the volume near the exterior surface. The typical range of density for an

Alporas foam sample is 0.18 – 0.24 g/cm3.

The compressive stress – strain behavior of the Alporas foam samples was mapped using a

quasi-static strain rate of 1 x 10-3 s-1, as shown in Figure 20. Each sample has a curve

desirable in a metal foam. Its long plateau region indicates that there is a large capacity for

energy absorption. This foam has a plateau stress of approximately 2 – 3 MPa and a

densification strain of approximately 60–70%. The reported energy absorption per unit

volume at a strain of 50% for the smaller pore size material is 1.32 MJ/m3 and for the larger

pore size material is 0.94 MJ/m3. A summary of material properties of Alporas foam is

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0 5 10 15 20 25 Fr e q u e nc y ( % )

0 2 4 6 8 11 12 14

Cell size (mm) Small Pore Size Alporas

Figure 17. Alporas Foam (A) With Mean Cell Size Of 3.0 mm, And A Cell Size Distribution (B) Ranging From 1-7 mm.

0 5 10 15 20 25 Fr e que nc y ( % )

0 2 4 6 8 11 12 14

Cell size (mm) Large Pore Size Alporas

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Figure 19. Apparent Density Distribution Of From The Cross Section Of (3) Alporas Foam Samples. The Apparent Density Range Is 0.18–0.24 g/cm3.

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Table 4. Physical And Mechanical Properties Of Alporas Foam.

Alporas Type (a) Alporas Type (b)

Pore Size (mm) 3 4.5

Density (g/cm3) 0.24 0.18

Relative Density (%) 8.9 6.6

Plateau Stress (MPa) 2.5 1.8

Densification Strain (%) 50 50

Energy Absorption @ 50% Strain (MJ/m3) 1.32 0.94

Strength / Density Ratio 10 10

Advantages/Disadvantages

The production process used to manufacture Alporas is a relatively simple method, capable

of producing large sizes and quantities of foam [23]. Complex part geometries are possible

using shape casting, whereby specific shape molds are used instead of standard rectangular

molds. For this type of manufacturing process, Alporas achieves a reasonably homogeneous

structure. However, it is difficult to control the density and pore size with foaming agents.

Additionally, use of additives such as calcium and titanium contaminate the aluminum matrix

material.

Cymat Stabilized Aluminum Foam

Cymat stabilized aluminum foam (SAF) is an ultra-light aluminum closed cell metallic foam

produced by the Cymat Corporation in Canada [7,14,27].

Production Process

The production process for Cymat SAF is shown in Figure 21. Aluminum is first melted in a

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This stabilizes the melt, preventing the pores from immediately collapsing. This mixture is

stirred until there is a homogeneous distribution of particles in the melt. The melt is then

poured into the foaming box. A gas, such as air, nitrogen, or argon, is introduced through a

spinning impeller designed to evenly disperse small gas bubbles. The cell size is controlled

by the gas flow rate, propeller design and the propeller rotational speed. As the foamed

material rises to the top, a conveyor belt draws a solidified foam sheet from the foaming box.

It is critical that the foam be handled gently so as not to damage the cellular structure of the

semi-solid foam. It is then cooled further and cut into usable lengths.

Characteristics

The relative density can be varied from 3-20% based on processing parameters. This would

yield densities between 0.08–0.54 g/cm3. Figure 22 is a photograph of a piece of Cymat SAF

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Figure 22. 90x90x50mm Cymat Foam, 10% RD.

with a relative density of 10%. Several stress / strain curves are shown in Figure 23 [28].

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Table 5. Physical And Mechanical Characteristics Of Cymat SAF.

Cymat SAF (A356)

Pore Size (mm) 3

Density (g/cm3) 0.4

Relative Density (%) 14.8

Plateau Stress (MPa) ~5

Energy Absorption @ 50% Strain (MJ/m3) 2.6 Strength / Density Ratio 12.5

Table 5 summarizes the physical and mechanical properties of Cymat SAF [29].

Advantages / Disadvantages

Cymat’s SAF production method is a low cost, relatively simple production method. The

starting materials are also low cost, commonly using recycled aluminum MMC scrap. Cymat

is capable of producing large pieces of SAF, usable in large scale projects like building

construction. Cymat’s process has good control of density on a macro scale, but the

product’s homogeneity has room for improvement. This process is less versatile at creating

shapes, as it is not foamable into a pre-molded shape. It is difficult to machine given the

delicate cell structure and the presence of silicon carbide particles often in aluminum MMC

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Experimental Procedure

This section will outline the experimental work done in this research thus far. The

processing steps for producing HSF samples will be summarized. Equipment specifications

will be given along with the procedure for its use. Any instrumentation settings will be given

for future reference. The raw materials used will be described including criteria for material

choice.

Equipment Used

The equipment used included a permanent casting mold, molding utensils, high temperature

furnace, linear precision saw, grinding and polishing stations, sonic cleaner, MTS

compression testing machine, optical microscope, and a scanning electron microscope (SEM)

equipped with energy dispersive x-ray spectroscopy (EDX).

Casting Mold

The permanent casting mold used for producing samples was first designed and then built in

the NCSU Mechanical and Aerospace Engineering Departmental machine shop.

Mold Design

The objectives in mold design were:

• Simple mold design for ease of fabrication.

• Uncomplicated design requiring no heaters, cooling channels, or external

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• Small size capable of fitting into the high temperature furnace for preheating.

• Easy disassembly for part removal.

• Built for repeated use.

• Effectively feeds molten metal to the part, and filters out any debris in the melt.

• Modular design that allows for easy modification after feedback from trials.

A36 steel was chosen for the mold. It is a construction grade carbon steel. The advantages

to this are its low cost, and the carbon content reduces potential bonding between the

aluminum part and steel mold walls. The growth rate of the Fe-Al intermetallic compound

layer decreases with increasing carbon content in the steel substrate [30]. The entire mold

was designed so that it could be fabricated with ¾ inch plate stock. One disadvantage to this

material choice is the weight penalty. Oxidation is also a potential issue. However, boron

nitride is used to coat all surfaces of the mold, serving two purposes. It acts as a mold release

agent allowing for easy part removal, as well as protecting the surface of the steel from

oxidation.

The design was modeled after a sand casting mold [31]. Sand casting molds are gravity fed

from a crucible of molten metal. The standard features of a sand casting mold that were used

included a sprue, runner, ceramic filter, and a riser. Renderings of the mold design can be

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a) b) c)

Figure 24. HSF Mold In Various Stages Of Assembly (A) Runner With Gate, (B) Adding The Filter And Side Walls, And (C) Fully Assembled.

Stainless steel bolts were coated with boron nitride spray and used to hold the mold together.

Unlike a sand casting mold, this mold needed to be used multiple times, so a nondestructive

method of part removal was needed. A slide shut-off gate was implemented to separate the

b) a)

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runner system and the mold cavity once the mold was filled. The side view in Figure 25 (a)

shows the molten aluminum path. Liquid metal is poured into the top of the sprue, it travels

through the runner under the top plate, goes through the gate and up through the ceramic

filter, then fills the cavity, and finally rises above the cavity into a riser area, used to feed any

shrinkage during solidification. The ceramic filters used were donated by Corning, Inc.

Each is 43 x 43 x 9 mm, and has 100 cells per square inch.

The mold size was chosen such that several samples could be attained from each cast HSF

block. It is said in literature [32,33,34,35] that to obtain statistically valid tests, there must be

a minimum of 8–10 cells per side of the foam test sample. This length is needed to eliminate

any sample edge effects, such that the data obtained from the sample test piece can be

extrapolated to larger pieces of bulk foam. The minimum length dimension for the mold was

chosen to be 54 mm. With a sphere diameter of approximately 4 mm, this length still allows

for half a sphere to be cut from each face, exposing the cell structure. The remaining 50 mm

is representative of approximately 12 spheres. This judgment is also validated by the HSF

research done by Georgia Tech, showing that a λ/d ratio of at least 10 produces repeatable

results.

Molding Tools

Necessary items were purchased for the casting process, including crucibles, tongs, and

safety clothing. The crucibles are made of fused silica (white), or graphite (dark gray),

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Figure 26. Molding Tools Used In Casting, Including Tongs, Crucible, And Quartz Stirring Rod.

equipment procured included a full face shield, aluminized apron and gloves, and protective

arm sleeves. Figure 26 shows some of the tools used.

High Temperature Furnace

A high temperature furnace, capable of heating to 1700°C, was used to preheat the mold and

melt the aluminum. Its interior heating chamber dimensions are nominally 12 x 12 x 12

inches. Its automatic controls allow for maintaining a constant temperature while monitoring

the percentage of power used to maintain that temperature. When melting metal or heating a

mold, it is useful to know when the contents have stopped absorbing heat, which is indicated

by a leveling off of power input to the elements. This starts off high, but eventually settles

on a value representative of the loss factor of the system itself.

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Figure 27. Buehler Isomet 4000 linear precision saw.

A diamond tipped wafering blade was used to wafer cut the edges off foam samples and to

partition samples. It has adjustable blade speed up to 5000 rpm, and a variable feed rate as

low as 0.05 inch / minute. Utilizing the slow feed rate assures a clean edge. A cutting fluid

is circulated to maintain low blade and sample temperatures and lubricate the cutting area.

Typical machine settings used when cutting samples were 0.2in/min feed rate and 4000 rpm.

Grinding and Polishing Stations

Samples were prepared for optical and SEM imaging using the Buehler Automet 2 Power

Head systems, as shown in Figure 28. The rotating grinding/polishing discs can be used

alone to manually prepare samples, or the automatic head can be used to automatically polish

samples.

Larger pieces were ground and polished by hand. The compression testing samples were

processed using a range of grinding papers from 180 grit to 1200 grit. A stream of water was

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100 rpm, based on the stage of grinding, using lower speeds for final finishing. Grinding of

larger samples was done to create a flat, scratch free surface for digital camera imaging

during compression testing.

Smaller samples intended for SEM imaging were mounted in epoxy. These samples can then

be set into a blank holder that connects to the power head. The power head allows for

adjusting the amount of pressure exerted on the samples, as well as rotating the sample

against the polishing disc. These mounted samples were first ground using a series of papers

from 180 to 1200 grit. After grinding, these samples were polished using a polishing wheel

with diamond slurry (9µm, 6µm, and 3µm particle size), and then with alumina paste (0.05

µm particle size). The wheel rotational speed varied between 60 – 80 rpm, based on the

stage of polish, using lower speeds for final finishing. When using the powerhead, the force

was set at 4 – 5 pounds, the lowest usable setting on the machine.

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Figure 29. Buehler Ultramet 2002 Sonic Cleaner.

Sonic Cleaner

Another useful tool is the Buehler Ultramet 2002 sonic cleaner, shown in Figure 29. It has a

3 liter stainless steel tank and a 47kHz transducer. Samples were cleaned in water, acetone

and alcohol to remove debris and residual polishing compound. Samples were typically

cleaned for a minimum of 5 minutes.

Compression Testing Machine

The Construction Facilities Laboratory (CFL), within NCSU Civil Engineering, provided use

of their large tonnage compression testing machine. Compression testing was conducted

using their MTS-810 universal testing machine. It has a 220-kip load cell and closed-loop

control. The crosshead speeds used were 0.5mm/min for the first specimen and 1.25mm/min

for the second specimen. Additionally, a block of solid aluminum was tested using a

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Figure 30. 220-Kip MTS-810 Closed-Loop Universal Testing Machine.

second intervals. Figure 30 shows the MTS machine in the CFL.

Optical Microscope

An optical microscope was used to examine the surface of the polished samples. It was

important in polishing to remove all scratches and oxides from the surface for future

microscope imaging. The microscope is a Unimet Unitron 9279 optical microscope with

magnification capability of 5 – 1000 X. It is equipped with a digital camera, image

grabbing and image processing software on a dedicated computer station. A picture of this

microscope is shown in Figure 31.

Scanning Electron Microscope

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Figure 31. Unimet Unitron 9279 Optical Microscope.

analysis was the Hitachi S-3200N available in the Analytical Instruments Facility (AIF) on

the NCSU Centennial Campus. Most images were captured using the secondary electron

detector. The backscatter detector was sometimes used to capture images showing high

contrast between different phases. Specimen current imaging was done to illuminate

porosity and voids in the samples. Energy dispersive x-ray spectroscopy (EDX) was done to

determine the compositional analysis. The analysis was primarily qualitative to determine

the various phases present in the sample and where each phase was located. This produces a

color map image showing the spatial distribution of elements. A quantitative analysis was

also done, making it possible to estimate the proportions of elements present and thus the

feature’s molecular formulas. However, SEM is not very accurate when attempting to

determine carbon and oxygen content. Typical settings used in analyzing the metal foam

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Table 6. Typical SEM Settings Used In Imaging HSF Samples.

Hitachi S-3200N SEM Settings Values

Accelerating Voltage 10 – 20 kV

Magnification 40 – 20,000 X

Working Distance 15 – 20 mm

Aperture 0 – 4

Beam Current 40 – 60 %

Raw materials

The hollow spheres obtained from Fraunhofer were made of low carbon steel. The

aluminum alloys used were 356 and 443 casting alloys. Dissimilar materials were chosen

due to the nature of the casting process. Since the aluminum is melted and cast around the

spheres, it is necessary that the spheres remain structurally stable when exposed to 700°C

temperatures. Therefore, the melting temperature of the sphere material must always be

lower than the melting temperature of matrix material.

Hollow Spheres

The hollow spheres used in this study were produced by Fraunhofer in Dresden Germany, as

shown in Figure 32. The spheres are made of low carbon steel, with a reported composition

of 0.002% carbon, 0.007% oxygen, and the balance iron. The reported outside diameter is

nominally 3.7 mm, with a distribution curve shown in Figure 33. The wall thickness is

approximately 200 µm with wall porosity of approximately 5%. Fraunhofer uses a powder

metallurgy process to make the spheres, as outlined previously in the section on Fraunhofer

HSF. The microstructure and porosity over the wall thickness have been studied using

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Figure 33. Fraunhofer Size Distribution Of Carbon Steel Spheres Produced For This Project.

Aluminum Matrix

Due to the nature of liquid metallurgy, the interstitial matrix material must have a lower

melting point than the sphere material, or the thermal stability of the spheres may be

compromised in preheating or casting. The aluminum alloys chosen were casting alloys 356

and 443. Each was chosen for its favorable casting characteristics [36]. Alloy 443 is a

casting alloy with silicon as its major alloying element. Due to the silicon content, it has

high fluidity in the molten state and excellent feeding characteristics during solidification.

Alloy 356 has added magnesium to increase strength and to make it heat treatable. Both are

customarily chosen for intricate castings requiring good strength and ductility. The chemical

compositions of the alloys as delivered are shown in Table 7. Minimal iron content is

desirable, as phases formed in combinations of Al, Fe, and Si reduce the strength of

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Table 7. Aluminum Alloy Compositions As Delivered By Trialco, Inc.

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

Alloy-356: 7.01 0.50 0.11 0.28 0.39 0.02 0.06 0.09

Alloy-443: 5.27 0.59 0.33 0.13 0.04 0.05 0.11 0.08

HSF Processing Method

Processing Steps

The steps used in processing the HSF samples are given below.

1. Each piece of the mold is cleaned and degreased. This is necessary to reduce

contamination to the part and promote good adhesion of the boron nitride mold

release to the surface of the mold.

2. All surfaces of each mold component are sprayed with boron nitride spray. Boron

nitride Aerosol Lubricoat is a high temperature, anti-stick release agent for various

metal processing applications, including the melting and casting of nonferrous metals

and alloys [37]. It consists of a 97% BN powder in an acetone/alcohol base. It serves

to coat the steel mold, while forming a non-wetting, non-reacting layer with molten

aluminum. It is also effective at minimizing oxidation of the steel mold surfaces

during mold heating.

3. The mold is then partially assembled, putting together the base of the mold, securing

the ceramic filter and lower screen, and attaching the walls. Figure 34 (a) shows the

partially assembled mold.

4. The cavity is filled with hollow spheres and these spheres are vibrated and packed

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34 (b) shows the mold fully assembled for a trial using BBs in place of hollow

spheres.

5. The entire mold is placed in the furnace. The temperature is set at 700°C. The mold

is allowed to come to temperature. The point of saturation is determined by

monitoring the amount of power supplied to the heating elements. The power

declines and eventually comes to an equilibrium setting, indicating the level required

to solely maintain the heat loss of the furnace itself.

6. Simultaneously, a casting aluminum alloy is melted in a crucible. The volume of

aluminum needed to cast the part is less than the capacity of the crucible used, but it

does not all initially pack into the crucible given the size and shape of the aluminum

stock. Therefore, after an initial amount of aluminum is melted, the door is opened to

feed pieces of aluminum into the crucible. A picture of the mold and crucible inside

the furnace is shown in Figure 34 (c).

7. Once the mold is completely heated and the aluminum is melted, both are removed

from the furnace. A picture of the preheated mold and aluminum is shown in Figure

34 (d) just prior to removal from the furnace.

8. Casting was done with two people, both holding the crucible with crucible tongs, one

on each side. Aluminum is poured from the crucible into the sprue tube. It flows

through the runner (bottom channel), through the gate and up through a ceramic filter,

and fills out the cavity, while pushing the air from the cavity.

9. Aluminum continues to rise into the free space above the spheres and upper screen,

creating a supply of liquid aluminum to feed any shrinkage.

Development of Closed Cell Metallic Foam Using Casting Techniques

Abstract

Dedication

Personal Biography

Acknowledgements

Table of Contents

List Of Tables

List Of Figures

List Of Symbols And Abbreviations

Introduction

Potential Applications

Research Objectives

Literature Review

Liquid Metallurgy Techniques

Processing

Characteristics

Processing

Characteristics

Processing

Characteristics

Advantages/Disadvantages

Production Process

Characteristics

Advantages/Disadvantages

Production Process

Characteristics

Advantages / Disadvantages

Experimental Procedure

Equipment Used

Mold Design

Raw materials

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

Alloy-356: 7.01 0.50 0.11 0.28 0.39 0.02 0.06 0.09

Alloy-443: 5.27 0.59 0.33 0.13 0.04 0.05 0.11 0.08

HSF Processing Method