1:30 PM
*R2.1
MECHANICAL BEHAVIOUR OF ALUMINUM
FOAMS. L.J. Gibson, Department of Materials Science and
Engineering and A.E. Simone, Department of Civil and Environmental
Engineering, MIT, Cambrige, MA.
The compressive
stress-strain responses of two closed-cell aluminum foams were
measured. One foam was made by mixing silicon carbide particles in
molten aluminum and blowing gas through the melt. The second foam
was made by mixing calcium in molten aluminum to increase its
viscosity and then adding powdered titanium hydride to the melt; on
heating, the hydride decomposes to form hydrogen gas. The mechanical
tests indicated that the Young's moduli and compressive strength were
substantially less than those expected from existing models. There
were a number of microstuctural features which contributed to the
reduction in properties: variations in density and anisotropy,
curvature in the cell walls and corrugations in the cell walls.
Finite element models were developed to estimate the magnitude of the
stiffness and strength reduction associated with each feature acting
independently. The results of the models are consistent with the
reductions in the measured properties.
2:00 PM
R2.2
COMPRESSIVE, TENSILE AND SHEAR
TESTING OF MELT-FOAMED ALUMINIUM. Heiko von Hagen,
Wolfgang Bleck, Aachen University of Technology, Institute of Ferrous
Metallurgy, Aachen, GERMANY.
For structural
applications, materials have to provide mechanical properties that are
reliable for the user. Therefore the most suitable material
parameters have to be determined for today's foams by material's
testing and evaluation. The overall important influence of the foam
density is mostly understood, but for construction purposes it seems
to be important to get information about the possible influence of
other parameters like the sample thickness on the mechanical
properties and the deformation behaviour. Differences in the chemical
composition of melt-foamed aluminium material are important for
applications because they result in more ductile or more brittle
failure mechanisms. The behaviour of aluminium foam samples under
compressive, tensile and shear loads can be examined in the special
testing methods for sandwich material as described in the German DIN-
or the ASTM-standards. The test procedures according to DIN have been
established at the Institute of Ferrous Metallurgy for the testing of
metal foams and sandwich material (steel facesheets and aluminium foam
core). A great number of aluminium foam samples of discontinuously
processed melt-foamed material (Shinko-Wire) was tested to get
information about the mechanical properties depending on the variation
of density (0.2 to 0.43 g/ccm) and thickness (10 to 30 mm). The aim
is to provide a complete database of the resulting mechanical
properties in compression, tension and shear. Visualizing the results
the most reliable and suitable material parameters regarding
structural applications can be defined. Additionally samples of
continuously melt-foamed material were tested to get information about
the differences in mechanical properties and deformation based on the
chemical composition and the foaming process.
2:15 PM R2.3
THE
EFFECTS OF THERMOMECHANICAL PROCESSING ON THE RESULTING MECHANICAL
PROPERTIES OF 6061 ALUMINUM FOAM. R.W. Margevicius, P.W.
Stanek, and L.A. Jacobson, Los Alamos National Laboratory, Los
Alamos, NM.
The mechanical properties of foamed
materials depend not only on such obvious factors as constituent metal
or alloy and porosity but also on more subtle factors such as the
processing method. Current foaming processes typically produce
cellular materials that are approximately 10% of theoretical density,
and making a foamed material with a higher density is not
straightforward. The aim of the Los Alamos program is ultimately to
produce beryllium and beryllium alloy foams that have relative
densities higher than 10%. Thermomechanical processing is being
evaluated as a potential means to increasing the density of
conventionally produced foams. To gain an initial understanding of
the effects of compaction of the 10% foams, 6061 aluminum foam was
used as a surrogate for beryllium. Foamed alloy was obtained from a
commercial vendor in the form of a 100x300x300 mm (4x12x12 inch) cast
billet with a pore size of approximately 0.8 mm (20 pores per inch).
Increasing the density of the foam was achieved by uni-, bi-, and
triaxial compaction (compression, rolling, and HIPing) in the 25-500ƒC
temperature range. Metallography and room temperature mechanical
testing were performed to characterize both the starting and processed
material. Results will be presented which critically compare the
processed material to the starting material in terms of their
microstructures and mechanical behavior (compression, toughness, and
fatigue strength).
2:30 PM
R2.4
Abstract Withdrawn.
3:15 PM
*R2.5
COMPRESSIVE DEFORMATION AND
YIELDING MECHANISMS IN CELLULAR Al ALLOYS DETERMINED USING X-RAY
TOMOGRAPHY AND SURFACE STRAIN MAPPING. Hillary
Bart-Smith, Ashraf F. Bastawros, Daniel R. Mumm, Anthony G. Evans,
Harvard University, Division of Engineering and Applied Sciences,
Cambridge, MA; David J. Sypeck, Haydn N.G. Wadley, University of
Virginia, School of Engineering and Applied Science, Charlottesville,
VA.
The mechanisms of compressive deformation that
occur in both closed and open cell Al alloys have been established.
This has been achieve by using x-ray computed tomography (CT) and
surface strain mapping to determine the deformation modes and the
cell morphologies that control the onset of yielding. The deformation
is found to localize in narrow bands having width of order of a cell
diameter. Outside the bands, the material remains elastic. The cells
within the bands that experience large permanent strains are
primarily elliptical. Conversely, the cells that remain elastic are
equiaxed, regardless of their size. The implications for
manufacturing materials with superior mechanical properties are
discussed.
3:45 PM R2.6
SHEAR
PROPERTIES ON ALUMINIUM METAL FOAMS PREPARED BY THE MELT ROUTE.
Ernesto Saenz, Pedro S. Baranda, UTRC, SPAIN; Jorge
Bonhomme, ITMA, SPAIN; Petter Asholt, Hydro Aluminium, NORWAY.
Tensile shear testing were performed on one alloy of
Aluminium metal foam with different densities, supplied by Hydro
Aluminium, to determine shear modulus and shear strength. The
densities were 0.19 g/cm3 and 0.31 g/cm3. The base material of both
alloys contained 15% (vol) of 13um SiC particles. Four panels of
each density were tested according to ASTM C 273-61. The specimens
were initially bonded on Aluminium plates but due to the plates
deformation during the shear test they were bonded to steel plates.
The relative displacement of the plates was measured using two
extenssometers. Furthermore, with the objective to analyze the
possible effect of the cell size distribution on shear properties,
cell size and material distribution analyses were done in areas close
to the samples. At high densities a fast failure was observed after
the maximum shear load while at low densities the failure was
progressive. The failures in both materials were located in areas
with least number of cells.
4:00 PM R2.7
BENDING
PROPERTIES OF FOAMED ALUMINUM PANELS AND SANDWICHES.
Frantisek Simancik, Jaroslav Kovacik, Natalia
Sralliakova, Institute of Materials and Machine Mechanics SAS,
Bratislava, SLOVAKIA.
Panels of aluminum foams are
much stiffer than bulk aluminum sheets of the same weight. This is
because a more preferable distribution of the metal along the neutral
bending axis resulting in a higher cross sectional moment of inertia
for the aluminum foam compared to the bulk Al-sheet. Since the foamed
panels are usually covered by a dense aluminum skin they can easily be
used in various structural applications where high stiffness at a low
weight is essential. Experimental samples with a typical density of
0.5-0.8 g/cm3 were prepared from both cast and wrought aluminum
alloys via the powder metallurgical route. In this case aluminum
alloy powder is mixed with a foaming agent and continuously extruded
into a foamable wire-shaped precursor. The precursor is then arranged
in a special furnace and heated up to the melting temperature and
foamed into panels up to 1000*1000 mm and 5 - 50 mm thick. Sandwiches
with a foamed aluminum core were prepared simply by foaming cast
aluminum alloy between two aluminum face plates. After the foaming,
the face plates are diffusion bonded with the foamed core. This type
of bonding enables a formability of the sandwich plate and results in
a significant improvement of its mechanical properties and its thermal
stability. Bending stiffness of the samples was determined by the
four-point bending test. The influence of density, matrix
composition, pore size and thickness of the foamed plate on its
deflection or fracture was investigated. Up to the maximum possible
deflection, limited by the testing tool, only cast alloy based foams
have fractured. The samples based on wrought alloys as well as
sandwiches have survived this deflection without significant cracks in
the structure. It will be shown that the modulus of elasticity of the
foam, which depends on its density, cannot be used for the calculation
of the bending stiffness of the plates. This is because the apparent
density of the plate is influenced by the surface skin on the foam,
especially at small thickness. Therefore the cross-sectional moments
of inertia were modelled for various square weights of foamed
plates.
4:15 PM R2.8
HIGH
CYCLE FATIGUE PROPERTIES OF ALUMINIUM FOAMS. Bernhard
Zettl, Stefanie Stanzl-Tschegg, University of Agriculture, Institute
for Meteorology and Physics, Vienna, AUSTRIA; Rudolf Gradinger,
LKR-Centre of Competence on Light Metals, Ranshofen, AUSTRIA; H.
Peter Degischer, Technical University Vienna, Institute of Material
Science, Vienna, AUSTRIA.
Aluminium foams produced
from powder metaluurgical prepared precursor material have a high
potential for use in weight sensitive construction parts. The
relative density of the foamed Al-Si-Mg wrought alloys and Al-Si
casting alloys is in the range of 10 - 30% of the bulk density.
Additionally, these materials show excellent energy absorbing
properties and therefore are appropriate to be used in vehicle body
parts (``light metal construction'', ``crash energy absorber'').
The governing parameters for the practical design of a component are
based on the strength properties of the construction material. In the
present work, the Static mechanical properties and deformation
behaviour has been determined by tensile, compression and bend tests.
Under compression loading, the stress-strain curves show a pronounced
plateau-stress, which is correlated with the initial mass density of
the material by a power law dependence.
Construction parts in automobile industry, for example, are often
subjected to a high nunber of varying stress amplitudes. Therefore
the fatigue properties must be known for a reliable use of this
cellular material. Since load sequences of typical automotive
components may consist of 100 million cycles or more, the fatigue
properties in the high cycle fatigue range are of special interest.
The fatigue properties of Aluminum foams were investigated using a
high frequency fatigue testing method in the present study.
Specimens are subjected to a resonance vibration at a frequency of
about 20 000 Hz. Fatigue measurements in the range between 10 000 and
1 billion cycles result in a bent SN-curve.
The fatigue properties of different Aluminum foams were correlated
with the structure of the material described by quantitative image
analysis of optical micrographs. The distribution of in the crack
initiation area was determined using X-ray computer tomography.
Electron microscopical studies served to characterise the crack
initiation mechanism.
4:30 PM R2.9
STRAIN
RATE EFFECTS IN POROUS MATERIALS. James Lankford, Jr. and Kathryn A. Dannemann, Southwest Research Institute, San Antonio,
TX.
Cellular metals are under consideration as
potential energy absorbing structural components. The deformation
capacity of these lightweight materials allows for significant energy
absorption under blast or impact type loading. This paper will
provide an assessment of the behavior of metal foams under rapid
loading conditions. Dynamic loading experiments have been conducted
in our laboratory using a split Hopkinson pressure bar apparatus and
a drop weight tester; strain rates ranged from 45 s-1 to 1000
s-1. The implications of these experiments on open-cell, porous
metals, and closed- and open-cell polymer foams will be presented.
It will be shown that there are two contributors to the impact
resistance of cellular metals: (i) elastic-plastic resistance of the
cellular metal skeleton, and (ii) the buildup of gas pressure within
closed-cell structures, or the air pressure generated by airflow
within distorted open cells. A theoretical basis for these
implications will be discussed.
4:45 PM R2.10
THE
EFFECT OF PARENT METAL PROPERTIES ON THE PERFORMANCE OF LATTICE BLOCK
MATERIALTM. Mark L. Renauld, Anthony F. Giamei,
Mark S. Thompson, United Technologies Research Center, East Hartford,
CT; Jonathan Priluck, JAMCORP, Wilmington, MA.
Lattice Block Material or LBM
is a unique light-weight
structure consisting of repeated cells with an internal node
connected to 14 ligaments. In its metallic version, this product is
available in a variety of castable metals including aluminum alloys,
copper alloys, nickel alloys and steels. The relationship between
LBM structural performance (strength and stiffness) and
parent metal properties is investigated using compression tests in
three primary orientations and 3-pt. bend tests. Analytical
assessment of the LBM via finite element analysis shows
reasonable agreement with experimental findings and provides
predictions for LBM capabilities with different materials,
unit cell sizes and ligament geometries.
This work is supported by an ONR contract, Number N00014-95-C-0231,
and a DARPA/ONR contract, Number N00014-96-C-0400, both of which we
monitored by Dr. Steven S. Fishman.
SESSION R3: MANUFACTURE OF SOLID FOAMS
Chairs: John
Banhart and Donald S. Shih
Tuesday Morning, April 14, 1998
Nob
Hill A
8:30 AM
*R3.1
PRODUCTION METHODS FOR METALLIC
FOAMS. John Banhart, Fraunhofer-Institute for Applied
Materials Research, Bremen, GERMANY.
The
possibilities for making metallic foams or porous metal structures in
general are reviewed The various processes are classified according to
the state of the starting metal - liquid, powdered or ionised. Liquid
metal can be foamed directly by injecting gas, gas-releasing foaming
agents or by producing saturated metal-gas solutions. Indirect
methods include investment casting and usage of filler materials.
Metal powders can also used as starting materials for metallic foams:
mixtures of such powders with foaming agents are compacted to foamable
precursor materials which can be foamed in a second step. Instead of
foaming agents inert gas can be directly entrapped in the precursor
material. Metal foams can also be foamed from metal powder slurries
or by using polymer/powder feedstocks. Finally, galvanic
electrodeposition allows to make metal foams with open pores. The
various methods are compared with respect to physical properties,
application range and costs.
9:00 AM
R3.2
ALUMINUM FOAMS ``ALPORAS'' THE
PRODUCTION PROCESS, PROPERTIES AND APPLICATIONS. Masao Itoh,
Tetsuji Miyoshi, Shinko Wire Company Ltd., New Technology
Production Div., Amagasaki, JAPAN.
In 1986. we
started to develop industrial production of ALPORAS, foamed aluminum.
After a decade, in 1997 we produced 2.000 cubic meters of the foamed
aluminum and sold 60,000 pieces of ALPORAS plates which amounts to
72.000 square meters. ALPORAS is made from the molten aluminum mixed
with calcium oxide as thickening agent and Titanium Hydride as blowing
agents. The foam is made of multi-sided closed cell. The relative
volume of ALPORAS is about 13 times solid aluminum The mechanical
strength and thermal conductivity are 1/100 of solid aluminum,
respectively. On the other hand, the unique foam shape yields the
useful physical properties that offers many functional application
from such as for acoustic absorption, energy absorption, relaxation
for sonic boom, electromagnetic shield, architectural material and so
on. Our paper highlights the development of production process, the
physical properties and the application of ALPORAS .
9:15 AM R3.3
THE
EFFECT OF OXIDE LAYERS ON GAS-GENERATING HYDRIDE PARTICLES DURING
PRODUCTION OF ALUMINIUM FOAMS. V. Gergely, T.W. Clyne,
Cambridge Univ, Dept of Materials Science, Cambridge, UK.
Melt routes to metallic foam production offer
attractions of low cost and the potential for good microstructural
control. In situ gas generation may be preferable to external gas
injection in terms of the important objective of generating a fine
and uniform cell structure. The main difficulty with this approach
has been that of ensuring that the gas-generating powder is suitably
dispersed throughout the melt before the gas is released and the
cells are formed. In the present paper, procedures are outlined for
preparation of powders for use in aluminium melts, where gas will be
released only after a suitable delay, allowing the powders to first
become well-dispersed in the melt and solidification to start.
Titanium hydride powders have been heat treated in air and other
atmospheres. The oxide formation which occurs under these conditions
has been monitored using thermogravimetric and differential thermal
analysis techniques. In this way, a study was made of the kinetics
of hydride decomposition and the formation of titanium dioxide. The
reaction of oxidised powder produced in this way with liquid
aluminium alloys was then studied by monitoring the rate of gas
release within an enclosed cell connected to a flow meter. This work
was supplemented by microstructural examinations. It was found that
the presence of even a relatively thin oxide layer on the surface of
the hydride particles induced an appreciable delay in the onset of
reaction between the Al melt and the titanium hydride. This is
consistent with previous information concerning the resistance of
titanium dioxide to attack by molten Al.
This information was then used in the design of foam production
experiments, using oxidised hydride powder. Small quantities ( a
fewÝg) of powder were injected in Al melts ( Ý1Ýkg) and dispersed by
mechanical stirring. The melts were then allowed to cool at a
controlled rate, such that hydrogen evolution occurred within the
system during a period when it was semisolid, generating a foam.
These experiments were carried out with and without the prior
presence of ceramic particles within the melt, these being designed
to increase the viscosity of the melt and thus inhibit bubble
movement. Studies were then made of microstructural features of the
foams, including measurement of the overall oxygen content. It was
found that low oxide content foams with relatively high void contents
( 50-60%) could readily be produced using this technique, without the
use of ceramic particles. The thickness of the oxide is shown to be
important in controlling both the void content and the pore
characteristics. Current work is aimed at further development of the
process, partly through modelling of bubble nucleation, growth and
coalescence phenomena.
9:30 AM
R3.4
INVESTIGATION FOR THE SELECTION
OF FOAMING AGENTS TO PRODUCE STEEL FOAMS. Chin-Jye Yu,
Harald Eifert, Fraunhofer Resource Center Delaware, Newark, DE;
Matthias Knuewer, Markus Weber, Fraunhofer Institute for Applied
Materials Research, Bremen, GERMANY.
During earlier
investigations conducted at the Fraunhofer Institute for Applied
Materials Research (IFAM), concentrated on the metal foaming
technology using the powder metallurgy process, it was shown that the
steel alloys are also foamable. To further investigate the
foamability of steel alloys, suitable foaming agents need to be
identified and characterized. This presentation will discuss the
thermal analyses of several potential metallic compounds for the
steel foaming. The foaming behaviors of the selected foaming agents
in the steel powder compacts will also be evaluated in terms of
various pressing methods and heating conditions.
9:45 AM R3.5
FOAM
DRAINAGE. Stephan Koehler, Howard Stone, Department of
Engineering and Applied Science, Harvard University, Cambridge, MA.
One of the biggest impediments to making uniform
metallic foams is drainage occurring during the liquid state. An
equation has been proposed to model the dynamics of foam drainage
based on the interplay of surface tension, viscous dissipation and
gravitational forces (Verbist and Weaire, 1994). Based on this, we
discuss several generic dynamical aspects of drainage in liquid
foams, including the scaling of drainage times with surface tension,
bubble size, viscosity and gravitational force. We introduce a simple
model for the continuous foam casting process (Alcan process).
10:00 AM
R3.6
SOLIDS-STABILIZED EMULSION
PROCESSING OF MICROCELLULAR CERAMIC FOAMS. Jeffrey C.
Huling, Los Alamos National Laboratory, Materials Science and
Technology Division, Los Alamos, NM.
Amorphous
silica and crystalline mullite foams having the unique combination of
low density (5 - 25% theoretical), fine cell size (5 - 20m) and
nominally closed cell structure have been produced from
solids-stabilized emulsions using colloid and sol-gel chemistry.
These materials can take the form of thick films, extruded shapes or
cast monoliths. Applications include lightweight structures, thermal
insulation, filtration and low dielectric constant materials in harsh
thermal, mechanical and chemical environments. Aspects of foam
processing and properties related to the microcellular structure and
phase composition will be discussed.
10:45 AM
R3.7
COMPLEX-SHAPED FOAMED ALUMINUM
PARTS AS PERMANENT CORES IN ALUMINUM CASTINGS. Frantisek Simancik,
Institute of Materials and Machine Mechanics, Bratislava, SLOVAKIA;
Franz Schörghuber, Illichmann GmbH, Altmünster,
AUSTRIA.
A new injection-moulding technique has
been developed for the manufacturing of complex-shaped foamed
alumimun parts with a density of 0,5 - 1,0 g/cm3 and for a
variety of different shapes and sizes. The foam is prepared from
aluminum powder mixed with a foaming agent and continuously extruded
into a foamable precursor. The precursor is then heated to the
melting temperature of the alloy. This leads to the formation of a
liquid foam, which is then injected into the desired cavity. Metal or
even sand molds can be used as a cavity.
The foamed parts are covered by a dense aluminum skin which improves
the mechanical properties of the foam. The thickness of outer skin
can be increased by using a special casting technique, thus providing
a unique possibility for the production of three dimensional
sandwiches with isotropic core properites. Experimentally prepered
foamed samples are investigated in various cross-sectional areas with
respect to their apparent density, pore size, shape and orientation.
It has been found that these properties essentially depend on the
composition of the base-alloy of the foam and on the parameters of
the foaming process before and during injection.
Such complex-shaped foamed parts could replace sand cores which are
usually applied in a foundry to produce weight-saving cavities in a
casting. In this case, the foamed parts will remain in the casting,
thus saving high labor and energy costs for the removal of the sand
cores. With aluminum foam cores completely closed lightweight
sections can be produced in a casting. This leads to a significant
improvement of the mechanical, vibrational and acoustic properties
compared with the original hollow part. The higher weight of the part
because of the presence of the foam can in most cases be compensated
because of the possible reduction of the outer wall
thickness.
11:00 AM
R3.8
INVESTMENT CAST NEAR NET SHAPE
COMPONENT BASED ON CELLULAR MATERIALS. P. Stojanov, I.
Wagner, D. Dedecke, P.R. Sahm, Giesserei-Institut, RWTH Aachen,
GERMANY; U. Blank, N. Iliev, F. Nahrendorf, C.G. Stojanoff, LHT, RWTH
Aachen, GERMANY.
Metal foams provide an enormous
potential for application. Cellular structures with open porosity
attract special interest. The permeable periodical three dimensional
cellular structures with their particular physical, chemical and
mechanical properties define an innovative, multi-purpose function
material. Basic structure parameters are volume fraction and
porosity. The high ratios of surfaces to volumes open a wide range of
possible applications. They can be used for components in heat
exchangers, filters, catalyst surfaces, as weight saving constructive
elements, deformable energy absorbers ...
Since 1996 cellular metal materials were cast at the
Foundry-Institute of the Technical University of Aachen by applying
investment casting technique. The investment casting technique
enables the manufacture of complex near net shape components.
Composites consisting of shaped cellular structures and massive parts
can be realized in a one step casting process. After casting, the
mold material has to be removed carefully. Actually, aluminum alloy
based components for usage in a solid-gas absorption heat
pump/cooling machine were tested. Volume fraction, porosity and
geometry were optimized for the application.
The optimization of each of the process steps for a more economic
production and the adaptation for new applications are major aims of
the ongoing research work. The cooperation with users is very
important for developing shaped cellular structures with properties
which are required by customers.
The production of complex shaped components with smaller pores and
defined volume fraction will be checked out. Removing the mold
material of fine porous structures without damaging the metal matrix
is an important key aspect.
11:15 AM
R3.9
JOINING OF ALUMINIUM STRUCTURES
WITH ALUMINIUM FOAMS. Joerg Burzer, Hans Wilhelm
Bergmann, Univ of Bayreuth, Dept of Materials Research, Bayreuth,
GERMANY.
The aim of this work is the evaluation of
new construction elements for applications in transportation industry
which are based on a combination of commonly applied aluminium
structures and aluminium foams. Due to the cellular structure of
metallic foams, these materials have high specific stiffness values
and energy absorption properties, making them suitable for
applications requiring light-weight structures. The development and
characterization of aluminium foams is the subject of several research
projects. Besides these investigations the development of processing
technologies is necessary for ensuring the further application of
foamed metallic structures. In addition to glueing, welding is a
possible technology for joining foams with semi-finished products.
Due to specific characteristics such as the keyhole effect and a
minimal heat input, the laser beam welding process can be used to
compensate for the existing lack of joining technologies. The work
includes the characterization of the joining process, the joining
mechanism and the mechanical properties of the joining zone. A
testing method for the joints is developed which is based on a common
tensile test in order to evaluate the influence of the main laser
welding parameters on the toughness of the joints and to afford a
comparision between laser beam welding and glueing process. The
question of the joining mechanism is investigated with the help of
metallographic studies. In addition, the stiffness and energy
absorption properties of aluminium hollows filled and joined with foam
structures are characterized.
11:30 AM
R3.10
LYOPHILIC LIQUID POROSIMETRY AND
A NEW AUTOPOROSIMETER. Ilya Tyomkin,TRI/Princeton,
Princeton, NJ.
Lyophilic liquid porosimetry
determines the volumes of different size pores by measuring the amount
of liquid in these pores, thus, providing pore volume distribution
(PVD) data for cellular, sintered, fibrous and other porous materials.
Unlike mercury porosimetry, it may use any liquid that wets the
sample. This method opens unique opportunities for porous structure
characterization. It provides realistic PVD analysis of materials,
when the liquid of interest effects (swells, shrinks, etc.) the porous
structure. It allows direct measurements of uptake and retention
capillary pressures at different liquid content in the sample. It
determines uptake/drainage hysteresis of real liquids. It determines
liquid/solid contact angles of different size pores within the sample.
It evaluates PVD of a sample surface, of an interlayer between 2
layers, of warp and/or fill yarns in a woven fabric, etc. It predicts
liquid partitioning between 2 specimens in contact. Unlike mercury
porosimetry, it can also be used for PVD analysis of both soft and
brittle materials. The new TRI/Autoporosimeter, version 97.1 has been
developed for automated PVD analysis. A pore structure analysis for a
wide variety of materials will be presented.
11:45 AM R3.11
EDDY
CURRENT CHARACTERIZATION OF METAL FOAMS. Kumar P.
Dharmasena, Haydn N.G. Wadley, University of Virginia, Department of
Materials Science and Engineering, Charlottesville, VA.
The structure of a cellular material is often
characterized by the relative density (or porosity), pore shape and
orientation, the average cell size, and the degree of pore
interconnectivity. The pore structure of the material is determined
by the production method and the processing conditions. The range of
densities and cell size variations attainable with these cellular
materials has created an interest in non-invasive sensor techniques
to characterize the foam structure. Here we report on the application
of an eddy current sensor to characterize Aluminum foam.
Multifrequency electrical impedance measurements were performed on
Aluminum samples of different relative densities and pore sizes. The
amount of metal contained within a given volume of foam varies with
relative density and cell size indicating a range of apparent
electrical conductivities which is a function of the metal fraction
of the foam and the tortuous paths for current flow. Low frequency
impedance data indicated relative insensitivity to pore size
variations enabling an independent measure of the relative density.
SESSION R4: THERMAL PROPERTIES OF SOLID FOAMS
Chair:
Joe K. Cochran
Tuesday Afternoon, April 14, 1998
Nob Hill A
1:30 PM R4.1
THE
EFFECTIVE THERMAL CONDUCTIVITY OF HIGH POROSITY FIBROUS METAL FOAMS.
V.V. Calmidi, R.L. Mahajan, Dept. of Mechanical
Engineering, University of Colorado, Boulder, CO.
This paper reports an experimental and theoretical study of heat
conduction in high porosity metallic foams. The aim of the study is
to investigate the effect of structure of the metal foam medium on
its effective thermal conductivity. While heat conduction in fully
saturated porous matrices has been studied in detail over the past
few decades, there is no known systematic study of heat conduction in
metallic foams. This study attempts to bridge the gap. Foams made
of aluminum alloy T-6201 in a range of porosities (0.88-0.98), and
pore sizes (5-40 ppi) have been used.
An experimental setup was designed to measure the thermal
conductivity of metal foams in air and water. The temperature
difference across the foam sample was measured for known values of
heat flux. The thermal conductivity was calculated using the slope
of the heatflux-temperature difference curve and the geometry of the
foam sample. The data thus
obtained indicates that although the thermal conductivity is a strong
function of the porosity, there is no systematic variation with pore
size. A correlation was developed based on the measured thermal
conductivity data for air and water.
An analytical model was also developed, based on the structure of the
metal foam matrix. In our model, this structure is a periodic
hexagonal array with the fibers forming the edges of the hexagon and
a square representing the intersections. An expression for the
effective thermal conductivity was derived for this structure. The
theoretical results thus obtained are in excellent agreement with the
experimental data for both air and water.
1:45 PM
R4.2
OPTIMIZATION OF OPEN-CELL
METALLIC FOAM HEAT DISSIPATION MEDIA. Ashraf F.
Bastawros and Anthony G. Evans, Division of Engineering and Applied
Sciences, Harvard University, Cambridge, MA.
The
performance of an open cell metallic heat dissipation media is
studied with forced circulating air. The influence of foam morphology
and heat sink dimensions on both the heat transfer coefficient and
the pressure drop are measured. The temperature profiles across the
foam block are monitored using an Infrared imaging system. The
measurements are used to establish the heat conduction
characteristics. Using these measurements, detailed comparisons are
carried out with a model based on the cross flow of a fluid over a
bank of cylinders. An optimum foam morphology is predicted from the
trends of the foam relative density, the cell wall diameter, d, the
heat sink thickness, b, and the air stream velocity,
v.
2:00 PM
R4.3
MODELLING AND TESTING OF FOAMS
UNDER HIGH TEMPERATURE LOADING. Jason Phan and A. Peter
Jardine, Military Aircraft Systems Division, Northrop Grumman Corp.,
El Segundo, CA.
Finite element models representing
aluminum foams have been developed to study the structural and
thermal behaviors of metal foams subjected to high thermal loads. The
models represent a plate containing a foam core of varying pore
density through a thickness sandwiched with two solid aluminum
facesheets. Solid elements are used to illustrate the temperature
distribution effect through the thickness of the plate. The
analytical procedure includes both thermal and structural analyses.
In the transient thermal analysis, aluminum thermal property
constants consist of thermal conductivity and the specific heat.
Variables in the models are the thickness of the foam core, the
porous density of the foam as it varies through the thickness, and
the heat transfer coefficient, which is dependent on the density of
the foam. The models are analyzed using a high temperature heat
source. Through conductive and convective heat transfer, the resulted
temperature distribution with respect to time is then applied to the
same FEM model as an external load condition to execute the
structural analysis. The aluminum structural properties, Young's and
shear moduli, are defiled as a function of temperature. The plate is
simply supported on one side to permit thermal expansion. The stress,
strain, and displacement are analyzed based on the variables defined
in the models.
Point-source and distributed heat loads were applied to one face of
foamed Aluminum samples and temperature distributions were recorded
from embedded thermocouples. The experimental results for several
densities of foam will be reported and compared to the FEM model
predictions. The degree of correlation will be reported for both
point and distributed thermal loads.
The authors gratefully acknowledge sponsorship from DARPA and ONR
under ONR agreement N00014-97-2-0001.
SESSION R5: OTHER FOAM MATERIALS AND PROCESSES
Chair:
John W. Hutchinson
Tuesday Afternoon, April 14, 1998
Nob Hill A
3:00 PM
R5.1
APPLICATIONS FOR SILICA-BASED
AEROGEL PRODUCTS ON AN INDUSTRIAL SCALE. Marc Schmidt,
Fritz Schwertfeger, Hoechst Research & Technology, Deutschland GmbH
& Co, Frankfurt/Main, GERMANY.
Aerogels, nanoporous
lightweight materials, were discovered more than 60 years ago. The
supercritical manufacturing process and expensive raw materials
typically used to produce aerogels prohibited an commercialization on
an industrial scale. Recently Hoechst developed a commercially
attractive ambient pressure production process which will allow
broader commercialization of aerogel products in the very near future.
Due to their unique combination of properties, aerogels have the
potential to find wide applications, especially in the field of
thermal insulation. In many cases aerogels can not be used as bulk
material. Many applications need, in addition to very low thermal
conductivity, other properties like mechanical stability or
flexibility which an aerogel monolith does not have. The property
profile of an aerogel containing product depends not only on the
properties of the aerogel itself but also on the properties of the
other components used, such as binders, fillers or additives. Only
the right combination of aerogel with these components will lead to an
attractive product which meets the needs of the end user. For thermal
insulation applications the paper will give some examples of aerogel
containing products which are very close to commercial production.
For translucent and opaque insulation problems, aerogel products with
different property profiles related to thermal insulation, sound
insulation and mechanical properties will be
demonstrated.
3:15 PM
R5.2
STRUCTURAL GRAPHITC CARBON FOAMS.
Kristen M. Kearns, Materials Directorate, US Air Force,
Wright Patterson AFB, OH.
Graphitic carbon foams are
a unique material form with very high structural and thermal
properties at a light weight. A process has been developed to
produce microcellular, open-celled graphitic foams. The process
includes heating a mesophase pitch preform in a pressurized reactor
to above the pitch melting temperature. At the appropriate time, the
pressure is released, the gas nucleates bubbles, and these bubbles
grow forming the pitch into the foam structure. The resultant foamed
pitch is then stabilized in an oxygen environment. At this point a
rigid structure exists with some mechanical integrity. The foam is
then carbonized to 800ƒC followed by a graphitization to 2700ƒC. The
shear action from the growing bubbles aligns the graphitic planes
along the foam struts to provide the ideal structure for good
mechanical properties. Some of these properties have been
characterized for some of the foam materials. It is known that
variations of the blowing temperature, blowing pressure and
saturation time results in foams of various open pore sizes, however
the mechanism of bubble nucleation is not known. Therefore foams
were blown with various gases to begin to determine the nucleation
method. These gases comprise of a variety of molecular weights as
well as a range of various solubility levels. The resultant
structures of the foam were examined and differences were noted to
begin to develop a explanation of the foaming
mechanism.
3:30 PM
*R5.3
TITANIUM AND STEEL HOLLOW SPHERE
FOAMS. K.M Hurysz, J.R. Clark, A.R. Nagle, C.U. Hardwicke, K.J. Lee,
J.K. Cochran, T.H. Sanders, Georgia Institute of
Technoloby, Materials Science and Engineering, Atlanta, GA.
Metal hollow sphere foams are fabricated by bonding
metal alloy hollow spheres at points of contact. The spheres are
formed from powder/acetone slurries using the coaxial nozzle process
to produce polymer bonded powder shells which are 2-3 mm in diameter
with a wall thickness of 100-150 microns. The spheres are dried in
free fall by evaporation of acetone to harden the walls. To produce
titanium alloy spheres, the starting powder is titanium alloy
hydride. Thermal treatment in an inert atmosphere decomposes the
hydride and sinters the titanium powder in the sphere walls to higher
than 96% relative density. Both titanium and Ti-6V-4V spheres and
foams have been produced. Oxygen contents are a concern for both
titanium compositions and processing is being altered to reduce
oxygen levels to increase ductility. To produce stainless steel
spheres, the starting powder is a mixture of iron and chromium oxide.
Thermal treatment in hydrogen reduces the oxides to Fe/Cr alloys with
less than 2% porosity in sphere wall. Steel compositions are of the
405 stainless type. Carbonization in CO/CO2 atmosphere followed
by heat treatment produces foams of either 410 or 420 type stainless
steels depending on carbon contents.
After metallic hollow spheres are produced, a closed cell foam can be
fabricated by bonding the spheres at points of contact. This provides
a pseudo-closed cell foam with continuous porosity at sphere
interstices. It has been shown that closed cell foams offer high
stiffness and strength. Hardness measurement on steel sphere walls
permitted estimation of yield stress which correlated to
stress/strain curves for individual steel spheres. When steel spheres
were bonded into foams, relative strengths were positioned between
open and closed cell models. This was encouraging because bonding in
the foams was less than optimum and the hollow sphere wall contained
defects. As processing improves, strengths should
increase.
4:00 PM R5.4
GAS
ATOMIZED HOLLOW POWDERS AND NOVEL POROUS STRUCTURES.
David J. Sypeck and Haydn N.G. Wadley, University of
Virginia, Department of Materials Science and Engineering,
Charlottesville, VA.
Gas atomized metallic powders
are regularly used throughout industry to fabricate cornplex shaped
parts exhibiting unique properties. During atomization, break-up of
the liquid metal stream can lead to bag formation and entrapment of
the inert flow field gas. The resulting hollow powders are an
unwanted by-product in the sense that they lead to porosity and
future sites of defect in solid parts. One can however, separate the
hollow powders and consolidate them in close packed arrays to form
novel porous structures. The partially compacted hollow shell
structures exhibit an improved metal distribution and a potential for
greater stiffness and strength than other porous structures at
comparable density. Very uniform cell morphologies with integral
bonding to dense alloy skins and/or superplastic forming to near net
shape are possible. Here, novel porous structures made from gas
atomized Ti-6Al-4V hollow powders are introduced. Methods for
increasing the proportion of hollow powders during atomization and
automating their separation are discussed.
4:15 PM
R5.5
FABRICATION AND PROPERTIES OF
SYNTACTIC MAGNESIUM FOAMS. Mark Hartmann, Robert F.
Singer, Univ of Erlangen, Dept of Material Science, Erlangen,
GERMANY.
The syntactic metal foams in the present
study consist of thin walled hollow alumina spheres that are embedded
in a magnesium matrix. These cellular composites were fabricated by
infiltrating a three dimensional array of randomly distributed hollow
spheres of uniform diameter with a metal melt by using a gas pressure
assisted casting technique similar to low pressure die casting. The
resulting material contains closed cells of homogeneous and isotropic
morphology at a volume fraction of around 62 %. By use of different
magnesium alloys and hollow spheres with varying relative wall
thickness the density of the foams can be varied between 1.0 and 1.4
g/cm„. The potential of the infiltration casting process to fabricate
metallurgically bonded sandwich structures with dense facesheets has
also been demonstrated. In order to evaluate the energy absorption
characteristics of these foams compression tests were carried out.
The syntactic foams showed compressive stress-strain behavior typical
for elastic-plastic foams with some indication of embrittlement. At
the beginning of the collapse plateau the stress-strain curve is
characterized by the appearance of a small peak stress with subsequent
minor fluctuations of the stress. This is due to a deformation
mechanism which is predominately characterized by a progressive
brittle failure of spheres along planes of maximum shear stress in
conjunction with plastic yielding and fracture of the matrix. As a
result of the narrow cell size distribution the plateau stress remains
virtually constant up to high strains of 50 to 65 %. The efficiency
in absorbing kinetic energy is very high and reaches 85 % for
optimized material. Using different Mg-alloys (cp-Mg, AM20, AM50,
AZ91, QE22) the effect of matrix strength on the compression strength
of the syntactic foams was investigated and seen to be of little
significance. On the other hand, an increasing wall thickness of the
hollow ceramic spheres leads to a significant strength enhancement of
the syntactic magnesium foams, while the specific energy absorption
capacity is maintained.
4:30 PM
R5.6
AL-ALLOY FOAMS PRODUCED BY
DIFFUSION OF METAL POWDERS. Ning Wang, Haydn N.G. Wadley
and Edgar A. Starke, University of Virginia, Dept of Materials
Science and Engineering, Charlottesville, VA.
Low-density metal foams with uniform pore structure have enormous
potential use in structural applications. A new powder metallurgy
method to produce such foams will be presented. In terms of the
Kirkendall effect, the rate at which the two types of atoms of a
binary solution diffuse is not the same. The element with the lower
melting point diffuses faster. Al powders are mixed with Zn powders.
The mixture is compacted and heated up to let Zn diffuse into Al
particles. Pores are essentially created by the diffusion of Zn. The
foam structures is characterized and the relationship between the
pore structure and the processing history is studied.
SESSION R6: HICE-BASED MATERIALS
Chair: Steven G.
Fishman
Wednesday Morning, April 15, 1998
Nob Hill A
8:30 AM
R6.1
DEVELOPMENT AND SCALE-UP OF THE
LOW DENSITY CORE PROCESS FOR TI-64*. D. Schwartz, D.
Shih, R. Lederich, R. Martin, D. Deuser, The Boeing Company, St.
Louis, MO; D. Barker, H. Gegel, UES, Dayton, OH, D. Zick,
Bodycote-IMT, Andover, MA; G. Cohen, LAI, Port Jefferson Station, NY.
Metallic sandwich structures consisting of solid face
sheets separated by a porous core are of interest to the aerospace
industry due to their high stiffness-to-weight ratios. Such
structures have potential as low cost alternatives to metallic
honeycomb-core sandwich structures, and will have higher temperature
capabilities than polymer-based sandwich structures. The Low Density
Core (LDC) process is a powder metallurgical technique for
manufacturing sandwich structures with porous cores. Inert gas is
first trapped in metal powders using a HIP consolidation technique.
The as-HIPed material can then be hot-worked using conventional
methods, e.g. hot-rolling, hot-pressing, or extrusion. A unique
feature of the LDC process is that thin porous-core sandwich sheets
(<3mm thick) can be produced. Subsequent annealing causes the
entrapped gas to expand, forming isolated, rounded pores throughout
the consolidated powder. Typically, porosity levels range from 20-40
vol.% in LDC materials. The effects of processing parameters on
Ti-64 LDC materials were studied in detail, focusing on the effects of
powder type, hot-working temperature and degree of deformation,
expansion anneal temperatures, and entrapped gas pressure. The LDC
process has been scaled up, and 2100 x 1300 x 4 mm3 sheets of Ti-64
have been produced by hot-rolling in a commercial facility. A variety
of mechanical tests have been conducted on the LDC Ti-64, including
tensile, bend, and buckling resistance tests. The results will be
summarized. Finally, future directions and novel concepts for
processing LDC Ti-64 will be presented. *This work was performed
under the aegis of DARPA and ONR, contract no.
N00014-95-2-0007.
8:45 AM
R6.2
SUPERPLASTIC FOAMING OF TiAND
Ti-6Al-4V. David C. Dunand, Department of Materials
Science and Engineering, Northwestern University, Evanston, IL;
Jacques Teisen, Department of Materials, Swiss Federal Institute of
Technology (ETH), Zurich, SWITZERLAND.
Solid-state
foaming of metals can be achieved by high-pressure compaction of
powders in presence of argon, and subsequent expansion of the
resulting high-pressure argon bubbles at ambient pressure and
elevated temperature. This 'controlled blistering' is particularly
attractive for titanium which cannot easily be foamed in the liquid
state, but it is limited by cell wall fracture due to the low
ductility of titanium. To solve this problem, we have investigated
foaming of commercial purity titanium (Ti) and of a titanium alloy
(Ti-6Al-4V) under superplastic conditions. Rather than using the
traditional microstructural superplasticity mechanism (requiring fine
grains which are difficult to achieve in porous powder-metallurgy
materials), we used transformation superplasticity (which occurs
independently of grain size by biasing of internal mismatch stresses
upon thermal cycling about the allotropic temperature). As compared
to control experiments performed under non-superplastic isothermal
conditions, superplastic foaming of Ti and Ti-6Al-4V leads to a
significantly higher pore volume fraction. This result is discussed
in light of the pore microstructure existing after
foaming.
9:00 AM
R6.3
FABRICATION OF LOW DENSITY CORE
AlBeMetR 162 STRUCTURES. Mark Svilar, J.M. Marder,
Brush Wellman Engineered Materials, Cleveland, OH; P.W. Stanek, Los
Alamos National Laboratory, Los Alamos, NM; and D.S. Shih, Boeing,
St. Louis, MO.
Aluminum - beryllium alloys can be
used for buckling-critical aircratt parts such as fuselage, because of
their excellent specific stiffness. For example,
AlBeMet162 alloys, with 62 wt.% Be
and 38 wt.% Al, have a room temperature modulus of 300 GPa and 2
gm/ml density. The weight saving could be further recognized by
producing porous AlBeMet162
materials. Low-density-core (LDC) sandwich panels with up to 20% of
porous AlBeMet 162 core have been produced using an entrapped argon
gas method. Various facesheets of Al6061, Al2024 and AlBeMet 162 have
been used in fabricating the LDC panels. Cans, made of the eventual
facesheet materials, filled win AlBeMet 162 powder, were evacuated,
then backfilled with argon gas, before sealing. The cans were then
hot isostatically pressed to near full density. This consolidation
process also trapped the gas and compressed the gas molecules in
discrete pores. Subsequently, the cans were conventionally hot rolled
into sheet prior to a thermal treatment that expanded the entrapped
gas into a porous core. Mechanical properties and potential aerospace
applications of the porous AlBeMet sandwich panel will be discussed.
*This study is sponsored by DARPA and ONR, contract no.
N00014-96-C-0398.
9:15 AM R6.4
LASER
ULTRASONIC CHARACTERIZATION OF THE ELASTIC PROPERTIES OF LOW DENSITY
CORE (LDC) Ti-6Al-4V SANDWICH STRUCTURES. Douglas T.
Queheillalt, Haydn N.G. Wadley, University of Virginia, Department
of Materials Science and Engineering, Charlottesville, VA; Daniel S.
Schwartz, The Boeing Company, St. Louis, MO.
A novel
powder metallurgy (P/M) technique has been recently developed to
produce lightweight, structurally efficient Ti-6Al-4V porous cores
for sandwich structures. These low density core (LDC) sandwich
structures are produced by pressurizing (3 - 7 atm.) a
Ti-6Al-4V powder compact with Ar gas and consolidating to a relative
density of 0.95 by hot isostatic pressing. The samples are then
hot rolled to create a finely dispersed distribution of pores. These
rolled structures are then annealed to allow the pores to expand due
to the entrapped high pressure Ar gas. This results in a porous
microstructure with a 0.60 - 0.80 relative density. In order to
design LDC structures, it is necessary to understand the
microstructural dependence of the elastic properties of these porous
materials. Laser based ultrasound has been used to measure the
longitudinal and shear wave velocities and hence the elastic
properties of LDC Ti-6Al-4V in the as rolled state and two expanded
states; annealed at 920ºC for 24hr and annealed at
1200ºC for 6hr. The laser ultrasonic data was compared with
several analytical models for the porosity dependent elastic
properties. A combination of laser ultrasonic data, X-ray diffraction
data and microstructural analysis indicate that the porous cores
produced by this novel P/M technique exhibit a relatively uniform
distribution of pores, no significant texture or preferred grain
orientation and the elastic properties are reasonably predicted by
the porosity dependent analytical models.
9:30 AM R6.5
EDDY
CURRENT AND LASER ULTRASONIC CHARACTERIZATION DURING EXPANSION OF LOW
DENSITY CORE (LDC) Ti-6Al-4V SANDWICH STRUCTURES. Douglas T.
Queheillalt, Bill Choi, Haydn N.G. Wadley, University of
Virginia, Department of Materials Science and Engineering,
Charlottesville, VA; Daniel S. Schwartz, The Boeing Company, St.
Louis, MO.
A novel powder metallurgy (P/M) technique
has been recently developed to produce lightweight, structurally
efficient Ti-6Al-4V porous-core sandwich structures. These low
density core (LDC) sandwich structures are produced by pressurizing
(3 - 7 atm.) a Ti-6Al-4V powder compact with Ar gas and
consolidating by a combination of hot isostatic pressing (HIP) and
hot-rolling. This process creates a finely dispersed distribution of
pores entrapped with high pressure Ar gas. These structures are then
annealed to allow the pores to expand and produce a porous core
structure with integrally bonded face sheets. A hybrid multifrequency
eddy current (EC) and laser based ultrasonic (LBU) sensor has been
developed to measure the expansion kinetics, density evolution and
elastic property evolution during the expansion of LDC Ti-6Al-4V
sandwich structures. The LDC samples were heated at a rate of
10ºC/min and held at 920C for up to 12 hr. The
eddy current sensor showed that the samples began to expand during
heating and expansion was nearly complete after 4 hr. The laser
ultrasonic sensor showed that there was also a concomitant decrease
in the elastic properties. Therefore, the hybrid eddy current and
laser ultrasonic sensor demonstrated that the density and elastic
property evolution can be independently measured during the expansion
of LDC Ti-6Al-4V sandwich structures. The data provides a good
starting point for the development of micro-mechanical based models
of the expansion kinetics of LDC Ti-6Al-4V sandwich
structures.
9:45 AM R6.6
PROCESS
MODELING OF TI-6AL-4V SPM MATERIALS. Shatil Ahmed, Gary
Huang and Harold Gegel, UES Software, Inc., Dayton, OH; Douglas
Barker, Materials & Processes Division, UES, Inc., Dayton, OH.
A new material density scheme has been developed and
incorporated into an existing FEM model for porous materials to study
the consolidation and forming processes of structurally porous
materials. To support the modeling of consolidation and forming
processes of Ti-6Al-4V SPM, material data as a function of
thermomechanical history was developed for establishing constitutive
relationships and intrinsic workability maps for billet materials
fabraicated by hot isostatically pressing an encapsulated mixture of
metal powders and argon gas by McDonnell Douglas Aerospace (Boeing).
Laboratory scale rolling experiments were conduted to develop
processing information needed for designing a full scale rolling
process. The subscale rolling process was modeled and the simulation
results compared to the experimental rolling results. The face
plates of the HIP can subsequently were the surface sheets of the
structurally porous material core. The computer simulation results
and processing maps, which describe the ran
10:00 AM R6.7
THE
INFLUENCE OF INTERNAL PORE PRESSURE ON THE ROLL FORMING OF
CLOSED-CELL METALLIC FOAMS. Dana M. Elzey and Haydn N.G.
Wadley, Department of Materials Science and Engineering, University
of Virginia, Charlottesville, VA.
An existing
plane-strain analysis of the rolling of a porous (compressible)
plastic core reinforced with dense (incompressible) face sheets has
been extended to include the influence of internal pore pressure.
Assuming a rigid-plastic face sheet obeying the Mises yield criterion
and a plastic core described by Doraivelu et alís yield function for
compressible solids, an effective yield potential is developed for a
face sheet/core sandwich. Void shrinkage during rolling leads to
increasing resistance to flow under conditions of high triaxial
compressive stress due to increased gas pressure within the voids
(assumed isolated). An upper limit on the attainable relative density
within the porous core is predicted as a function of the initial
internal pore pressure, roll size, coefficient of friction, face
sheet/core thickness ratio and plastic properties of the metallic
foam matrix.
SESSION R7: APPLICATION AND DESIGN PRINCIPLES
Chair:
Anthony Giamei
Wednesday Morning, April 15, 1998
Nob Hill A
10:30 AM *R7.1
ON
MINIMUM WEIGHT DESIGN OF FOAM-CORE STRUCTURES. Bernard
Budiansky and John W. Hutchinson, Harvard University, Cambridge, MA.
Basic principles of design for minimum weight are
reviewed for various kinds of structural components that incorporate
low-density foams as cores or sandwich fillers. The emphasis is
slanted toward metalliic foams and compression structures. Columns,
panels, and shells are considered, and weight comparisons are made
with competing structural configurations, such as hollow tubes,
stiffened flat panels, and stiffened shells.
11:00 AM
R7.2
PARAMETERS OF CONSTRUCTION FOR
APPLICATIONS OF METALFOAMS. Hipke Thomas, Fraunhofer
Institut Werkzeugmaschinen und Umformtechnik, Chemnitz, GERMANY.
The mankind has orientated by nature during its
technical development. For instance, the human skull is a
composite-construction with a relative strong and tight outer and
inner ply which is connected to a strong cellular structure. You can
also find this design in composite constructions of metalfoam.
The material is very interesting for using in machines and
handlingsystems, because it has an excellent loss factor for
mechanical vibrations and a very good specific stiffness. It is able
to design light weigth constructions for high dynamical applications,
for example:
- assemblies with highest acceleration (machine table, tool changing
system)
- long profiles wich have to high vibrations
- stiffening of different machine parts
Before metalfoam will be used succesfully there have to solve some
problems in the practical application. There are no certain knowledge
about the joining of different parts with metalfoam, the thermal
behaviour of composites or the structural damping of parts with
metalfoam inside.
The paper shows the results of analysis which was made in the
Fraunhofer Institut for machine tools and forming technologies in
Chemnitz. It was checked the strength of screw joints by different
base materials, foam densities and diameters. As screw joints were
used for example: metrical screws and nuts, woodscrews and screwed
bushes.
A second test include the measuring of deformation at composites by
different foam densities and materials of the outer plates. The
values was taken for global and lokal heating.
The dynamical behaviour of metalfoam is very good, because of it¥s
high material damping. The next task was to analyse this behaviour
when the foam is inside of a steel-tubes. There were used different
round and square tubes with different geometry.
11:15 AM
R7.3
MANUFACTURE AND APPLICATIONS OF
Ti AND Al STRUCTURAL POROUS MATERIALS*. D.S. Shih, D.S.
Schwartz, D. L. Buchanan, R.J. Lederich, R.L. Martin, L. Busche and
D.A. Deuser, Boeing, St. Louis, MO; B. Norris, Rohr, Chula Vista, CA.
Ti- and Al-based porous materials offer great
potential in weight and cost savings for aircraft structures. Using
an entrapped gas method, large low-density-core (LDC) sandwich panels
of Ti64 with 35% porous core have been produced up to a size
of -2,100 x 1,300 x 4 mm3. Component manufacture processes for the
LDC Ti64 sandwich panels were identified and demonstrated. They
included drape forming, superplastic forming, creep forming,
drilling, friction stir welding, and countersinking. MechanicaI
property and structural efficiency analysis were evaluated.
Manufacturing processes to fabricate aluminum foam-core sandwich
materials and structures were demonstrated. A prototype MD-900 NOTAR
tailboom was produced by rolling Al foam-core into a longitudinally
slotted tube of 610 mm in diameter and 600 mm in length. Sandwich
panels were produced by adhesive-bonding Al6061-T6 facesheets to
ALPORAS foam core. Several parts have been identified in Boeing
products as suitable candidates for manufacture using structural
porous aluminum and titanium materials. For porous Ti sandwich panels
they include: 1) outboard and inboard fuselage skins for the
F/A-18E/F, 2) engine pylon farings for the C-17, and 3) aft fuselage
panels for the JSF. Promising aluminum foam-core sandwich panel
applications include gun and electronics access doors for the
F/A-18E/F and cylindrical tailboom sections for the MD-900 NOTAR
helicopter.
* This study is sponsored by DARPA and ONR, contract no.
N00014-95-2-0007 and N00014-96-C-0398, and Boeing.
11:30 AM R7.4
LIGHT
WEIGHT PRODUCTS WITH METAL FOAMS-THEIR PROCESSING AND THEIR
PROPERTIES. R. Neugebauer, Chemnitz, GERMANY; H. Brunlich,
U. Wagner, FhG IWU, Dept Forming Technologies, Chemnitz,
GERMANY.
A goal of future technologies is to make
products lighter together with property optimizing and economically
facepoints.
The requirements of car and engineering industry concerning reduced
masses will be solved by metal foam. In this case fundamental
examinations about material behaviour and processing of metal foam
are necessary. From the properties of foam compounds new
possibilities as a light weight material can be deduced. Car and
aircraft engineering, space research, tooling engineering and other
industrial parts are potential possibilities for intelligent material
compounds with advantages in energy absorption capacity, sound
absorption and high bending strength if the material has a high
porosity.
The lecture will be give a review about properties and processing
parameters from selected foam compounds.
These are results from projects of Fraunhofer-Institut for Tooling
Machines and Forming Technologies together with the Saxon industry.
An Analyse of the industrial market will be based for an abstract of
technological standards for metal foam compounds. Examinations about
processing, mechanical and physical analyses and construction
parameters will be discussed.
SESSION R8: GASAR MATERIALS
Chairs: Daniel S. Schwartz
and Haydn N.G. Wadley
Wednesday Afternoon, April 15, 1998
Nob
Hill A
1:30 PM
*R8.1
FORMATION OF ORDERED GAS-SOLID
STRUCTURE VIA SOLIDIFICATION IN METAL-HYDROGEN SYSTEMS.
Vladimir Shapovalov, State Metallurgical Academy,
UKRAINE; Sandia National Laboratories, NM.
The
gaseous phase nucleation in formation of ordered gas-solid structures
may occure at the solidification front or on high melting particles
suspended in the liquid ahead of the front. The first mechanism would
seem more viable.
The crinical radius of bubble nuclei is shown to hyperbolically
decrease with pressure. The bubble nucleation rate is increased
accordingly.
Pore formation follows a mechanism district front that of evolution
of gas in carbonated beverages or bubbling in a boiling liquid.
Furthermore, the ellipsoidal pore growth cannot be ascribed to
stretching of gas bubbles by the advancing solidification front.
The pore growth proceeds by hydrogen transverse diffusion ahead of
the solidification front and concurrent advancement of the solid and
the gaseous phase into the melt. When the growth rates of the phases
are equal, an ideal structure including cylindrical channelways in a
nonporous matrix should form.
Unlike in solid-solid cutectics, the gasarite pore spacing is
determined not only by diffusional self-adjustment. Rather, it is a
function of the nucleation conditions and the solidification
pressure.
A pore may be terminated by any of the following causes:
- nucleation of a new pore in a spase between the growing
pores,
- supplementary removal of heat from the bubble tip via thermal
radiation into the channelway, and
- pressure reduction in the pore due to the gas average temperature
declining as extends.
Pore coarsening in growth of a gas cutectic may be caused by wedging
or pore coalescence. Wedging is mainly observed at low, and
coalescence at high void fractions.
Corrugated pores may form due to periodical changes of pressure above
the melt. Similar effects are caused by repeated detachment of
bubbles from the solidification front.
Pore orientation in gasarite is determined by the shape of the
solidification front, because the growth vector direction of a pore
is always normal to the front its location.
2:00 PM
R8.2
PORE/BUBBLE STABILITY IN GASAR
POROUS METALS PROCESSING. Jon Apprill, Univ. of
Arizona/Sandia National Laboratories, Dept. of Materials Science and
Engr., Tucson, AZ; Michael Maguire, Thomas Gutsch, Sandia National
Laboratories, Liquid Metal Processing Laboratory, Albuquerque, NM.
A novel approach to produce porous metals has been
developed in the Ukraine. These materials, termed GASARS by the
Ukrainians, are processed by melting under a large partial pressure
of hydrogen and then directionally solidifying. Hydrogen is driven
out of solution and grows as cylindrical pores with the
solidification front. The resultant structure resembles a laminar
eutectic, with pore diameters ranging from 100 um to 1 cm.
Experiments with GASAR Ni have been carried out under processing
conditions of varying hydrogen partial pressure, total pressure, and
superheat. Porosity in these experiments ranged from 6% to 63%. An
analysis was developed concerning pore nucleation and growth which
identifies conditions in which hydrogen bubbles are stable before
solidification. It is hypothesized that these conditions lead to low
porosity because these bubbles float out of the melt and escape the
advancing solidification front. The experimental results support this
hypothesis; processing conditions which favor bubbles exhibit low
porosity and visible bubbling while solidifying. This work was
supported by the United States Department of Energy under Contract
DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by
Sandia Corporation, a Lockheed Martin Company, for the United States
Department of Energy.
2:15 PM R8.3
THE
EFFECT OF GASAR PROCESSING CONDITIONS ON THE SIZE, SHAPE, DENSITY AND
DISTRIBUTION OF POROSITY IN ALUMINUM ALLOYS. C.J.
Paradies and A. Tobin, Northrop Grumman, AS&T, Bethpage, NY; J.
Wolla, Naval Research Laboratory, Washington DC.
Hydrogen was intentionally introduced into several different aluminum
alloys to evaluate the size, shape, density and distribution of the
resulting hydrogen porosity. Designed experiments were performed to
reveal the effect of processing conditions on the pore formation. The
final pore size appears to be defined by the hydrostatic pressure and
the hydrogen concentration in the melt. Bubbles nucleate at
heterogeneous sites and grow by diffusion of hydrogen from the melt
to the bubbles. Those bubbles that do not find their way to a free
surface become entrapped by the growing primary phase. The shape and
distribution of the resulting pores depends upon both the
microstructure of the solid and the time and location that the bubble
nucleated within the solid structure. The density of pores probably
depended upon a combination of factors including the density of
nucleation sites, the local supersaturation, the local solidification
time and the number of bubbles that found their way to a free
surface. The results of the experiments are analyzed and
discussed.
3:00 PM
*R8.4
CHARACTERIZATION OF POROUS GASAR
ALUMINUM. Robert Bonenberger, FM Technologies, Inc.,
Fairfax, VA; Richard K. Everett, Naval Research Laboratory,
Washington, DC; Anthony Kee, Geo-Centers, Inc., Fort Washington, MD;
and Peter Matic, Naval Research Laboratory, Washington, DC.
Experimental and numerical analyses were performed
on porous aluminum samples to evaluate microstructure and mechanical
properties. The samples were fabricated by the GASAR process, that
consists of a gas-solid eutectic reaction in which hydrogen is
allowed to diffuse into molten aluminum. Prior experimental and
numerical investigations involving copper GASAR metals have shown an
enhancement in yield strength with respect to solid copper for
porosity levels up to 25%. The present research is focused on
analyzing an aluminum alloy employed in structural applications.
Experiments were conducted on porous aluminum specimens to obtain
mechanical properties. The tests performed were tension, compression,
and torsion. Instrumentation on the specimens consisted of axial and
transverse extensometers for the tension and compression tests and
rotational variable differential transformers (RVDT's) for the
torsion tests. Properties measured included Young's modulus,
Poisson's ratio, shear modulus, and yield strength. Trends were
established for each of these properties as a function of the amount
of porosity in the sample.
Data from the experiments were used for creating finite element
models to investigate the effects ot constraint, pore arrangement,
solid zones, and gradients on the stress state of the material. A
3-D, multilayer model was constructed based on a hexagonal unit cell
containing a cylindrical pore, to study interactions between groups
of pores on the mesoscale. The model allows systematic spatial
positioning of the pore within the cell and the ability to fill the
pore with material, thereby forming a solid zone. Using hex cells as
building blocks, complicated microstructural arrangements can be
simulated.
3:30 PM
R8.5
METALLOGRAPHIC STUDY OF GASAR
POROUS MAGNESIUM. C. Park and S. Nutt, University of
Southern California, Department of Materials Science, Los Angeles,
CA; T. Donnellan and J. Papazian, Northrop-Grumman Corporation,
Bethpage, NY.
One of the promising techniques for
making porous metals is the so-called GASAR process, a method in
which a gas dissolved in the melt precipitates during solidification
to form pores. In principle, the process affords considerable control
over pore size, shape, and distribution. However, in practice, the
pore microstructure is difficult to control, and a clearer
understanding of microstructural evolution would be helpful. In this
study, we undertake a detailed microstructural study of a porous
magnesium alloy (AZ31) synthesized by the GASAR process. Three types
of pores were observed, distinguished by their size and morphology.
These included (1) large slighly elongated pores several mm in size,
(2) pore microtubules 50 m by a few hundred m in length,
and (3) fine equiaxed pores m. The types and distributions of
pores provide insight into the process of pore evolution. SEM and TEM
observations of the porous alloy will be presented, and the
microstructures will be related to individual process histories and
to pore evolution and control.
3:45 PM
R8.6
SOLUBILITY OF HYDROGEN IN MOLTEN
ALUMINUM ALLOYS FOR APPLICATION TO THE GASAR PROCESS.
Albert Tobin, Northrop Grumman Corp., Advanced Systems&
Technology, Bethpage, NY.
Aluminum alloys are
attractive materials for producing ultralightweight structural
materials via the GASAR process. Since the GASAR process depends
upon the rejection of hydrogen from the liquid during solidification
to form pores, the porosity of the solidified alloy will be strongly
affected by the extent of hydrogen solubility in the liquid metal.
Measurements of hydrogen solubility as a function of superheat
temperature and pressure were made in several commercial aluminum
alloys using a Sieverts apparatus and the effects of various alloying
additions were determined. In addition, the effects of silicon
boride nucleating agents were evaluated. Lithium additions to
aluminum alloys appeared to have the strongest effect in increasing
hydrogen solubility while Cu. Mg, and Si additions had much weaker
effects. These alloying effects will be discussed in terms of their
potential for producing low density structural materials using GASAR
technology.
4:00 PM R8.7
A MEMS
BASED HYDROGEN SENSING SYSTEM FOR GASAR APPLICATIONS. A.
Peter Jardine and Jason Phan, Military Aircraft Systems Division,
NOrthrop Grumman Corp. El Segundo, CA; David Haberman, DCH
Technologies, Sherman Oaks, CA; Michael C. Maguire, Liquid Metal
Processing Laboratory, Sandia National Laboratories, Albuquerque, NM.
GASAR is a casting technique for the production of
porous metals in which the porosity is due to the evolution of
Hydrogen bubbles during solidification. The successful implemental of
GASAR will provide a venue for innovative technique development for
reproducible measurements of the dissolved Hydrogen concentrations in
various non-hydride forming alloy melts. The measurement is
complicated by the high temperature environment of the molten alloy
and the large hydrogen over-pressures present in the melt vessel
required for hydrogen dissolution. An in-situ sampling scheme was
developed which pumps a Hydrogen-Argon mixture to a sampling manifold
in which a new MEMS based Hydrogen Sensor, mechanized as an
application specific integrated circuit (ASIC), is inserted. Details
of the MEMS based sensor will be described.
The objective of the work is to control batch producibility in this
complex metallurgical process. Calibration of the measurement system
will be reported based on known hydrogen concentrations and then in
comparison with conventional ex-situ measurement techniques located
at Sandia National Laboratory. Details of the sensing unit and its
incorporation into the Sandia GASAR chamber will be discussed
The authors gratefully acknowledge sponsorship from DARPA and ONR
under ONR agreement N00014-97-2-0001.
4:15 PM
FINAL COMMENTS